- The Characterization of the Chiral Prototype, Binapthol. The
structure and electronic spectrum of the chiral molecule binapthol is being
investigated to provide quantitative data to test theories relating optical
activity to molecular structure. Such theories are critical to the ability to
assess the chiral purity of a drug, particularly when the enantiomers of such a
drug can have dramatically different physiological effects. Concern over
possible or demonstrated negative side effects from one or more enantiomers of
a drug has accelerated the effort to market single enantiomer or chirally pure
drugs. Indeed, the chiral drug market is now a greater than 100 billion
dollar a year industry, with approximately 50 of the top 100 drugs
consisting of a single enantiomer. Despite the demonstrated importance of
chiral drugs, the detailed physics underlying the biomolecular interactions
responsible for their enantiomeric selectivity often remains obscure.
Of fundamental importance is the ability to determine the three-dimensional
structure of chiral molecules, and a long-used technique for structural
characterization has been to exploit the optical activity of chiral molecules.
The optical activity is conventionally quantified by measuring the ability of a
molecule to rotate the plane of linearly polarized light or by measuring the
differential absorption by the molecule of left and right circularly polarized
light through the technique of circular dichroism (Fig. 2).
Figure 2. The process of cavity ring-down enhanced
circular dichroism spectroscopy.
The validity of the approximate quantum mechanical models that relate these
measurements to molecular structure has not been rigorously tested. Using
molecular-beam high-resolution microwave and ultraviolet spectroscopies, we
have successfully characterized the molecular structure of the chiral prototype,
binapthol. Furthermore, we have recently modified the technique of cavity
ringdown spectroscopy to allow sensitive ultraviolet optical-activity
measurements. The combination of these spectroscopies will provide a rigorous
and unprecedented test of current theories that relate optical activity to
molecular structure and, in turn, assist efforts toward understanding the
biophysics underlying physiological chiral selectivity. (S. Davis and
D. Plusquellic)
- Optical Thermometry and Infrared Imaging Measurements for Biomedical
Applications in Dermatology. The development of laser-based treatment of
hair and skin requires quantitative measurements of their interaction with
laser light. Recently, the Gillette Research Institute in Gaithersburg, MD
requested the assistance of the Division to make such measurements using the
Division's capability in time-dependent infrared thermal imaging. In earlier
experiments, Gillette researchers laser-irradiated hair fibers and follicles
and found that for certain laser fluences and pulse durations, live extracted
hair follicles can absorb 800 nm light and become dormant—sometimes
remaining in a quiescent, non-growth state for up to several weeks. Gillette
and Division staff collaborated to quantify transient temperature changes
associated with laser heating and the heat dissipation processes of hair shafts
and live human hair follicles.
|
At NIST, temperature changes were measured in laser-irradiated hair fibers and
follicles of different pigments and textures using a liquid-nitrogen cooled
InSb (Indium Antimonide) IR focal-plane-array detector
(256 x 256 pixels)
to image the thermal emission from the samples. Pulsed irradiation of hair
follicles of different colors, under conditions that rapidly induce a catagenic
state in follicles (live but no hair growth), causes different maximum attained
temperature (T) based on hair color.
Diode-laser irradiation of follicles rich in the pigment eumelanin (black or
black/brown) produced IR emissions corresponding to temperatures T of
74 °C and 70 °C, respectively. Follicles with lower amounts of
eumelanin had a lower T: 62 °C and 50 °C for brown and gray
follicles respectively. Using the IR array detector, the decay of T
could be determined with 200 µs temporal and 25 µm spatial
resolution. T decayed exponentially with typical time constants of about
0.1 s (see Fig. 3). These results indicate that pheomelanin, present
in varying amounts in the lighter hair, does not absorb the 800 nm light
to the same extent as eumelanin and that the maximum temperatures attained and
profound structural damage observed in darker fibers are consistent with
explosive vaporization of melanin granules. This collaboration demonstrated
that this methodology is useful in measuring transient temperature changes in
microscopic samples with high temporal and spatial resolution. (L. Iwaki,
T. Heimer, and E. Heilweil, with P. Manos of Gillette) |
|
Figure 3. Thermal images of a laser-irradiated black hair fiber at times
relative to the start of the 50 ms laser pulse. The heat is observed to
propagate along the long axis of the fiber (horizontally in the image) at a
rate of roughly 3.5 mm/s. |
- Determining Absolute Polymer Orientation at Polystyrene Interfaces.
Vibrationally resonant sum-frequency generation (VR-SFG) measurements were
performed with a novel, broadband system developed at NIST [Opt. Lett.
23, 1594 (1998)] that allows rapid spectral acquisition. VR-SFG involves
the nonlinear mixing of an infrared photon, resonant with vibrations in the
sample, with a visible photon to produce a new photon at the sum frequency. It
is uniquely interface specific, as it is symmetry forbidden in centrosymmetric
media such as liquids or many solids (e.g., glass, silicon, bulk polymers).
|
We have used this technique to study the structure of polymer interfaces, since
polymer thin films have important roles in many industries, such as those
involved in semiconductor devices, optoelectronics, and tissue engineering. The
structure at both the free and buried interfaces of thin films is critical to
their performance as they can determine characteristics such as adhesion and
wear. Molecular characterization of these interfaces is difficult; conventional
probes using X-rays, neutrons, or linear vibrational spectroscopies do not have
the specificity to characterize the interfaces in the presence of the bulk
polymer, while VR-SFG does so successfully. The technique was demonstrated in a
study of the orientation of the phenyl rings, C6H5, at
the interfaces of polystyrene
(PS = (C2H3C6H5)n)
films spin-cast onto oxidized Si substrates.
Shown in the lower panel of fig. 4 are VR-SFG spectra for a series of
thin PS films. The features in the range 3000 cm-1 to
3100 cm-1 are due to the CH stretching vibrations of the phenyl
side groups. The modulation in the spectra is due to optical interferences in
the films. The upper panel compares the amplitude of the resonant component at
3067 cm-1 (points) to theoretical calculations (lines) for
phenyl groups located exclusively at the PS/air free surface, uniformly
distributed through the bulk, and at the buried PS/SiO2 interface.
The observed signal is clearly from the free surface, and detailed analysis
proves that the phenyl groups point away from the polymer film into the air,
with a tilt angle near 60° and a twist angle near 45°. This work was
the first to establish the complete orientational distribution for a pendant
side group at a polymer surface. In the future, similar optical interferences
will be exploited in multilayer systems (substrate/dielectric/polymer) to
characterize dielectric/polymer interfaces and explore the molecular basis of
adhesion. (K.A. Briggman, J.C. Stephenson, with L.J. Richter
(CSTL), and W.E. Wallace (MSEL)) |
|
Figure 4. Vibrationally-resonant sum-frequency-generation spectra for
thin polystyrene films ranging in thickness from 30 nm to 400 nm. |
- Light Scattering Ellipsometry. Light scattering is the primary
method employed by numerous industries for determining surface roughness.
However, measurement of the roughness of multiple interfaces of a dielectric
film by light scattering has not been achieved in the past, and there are few
non-destructive methods that can be applied. Meanwhile, ellipsometry of the
specularly reflected light is used ubiquitously to determine film thicknesses.
|
In a recent advance, the Division has shown that ellipsometry can be extended
to the scattering regime, yielding the power spectrum of the roughness of each
interface and the cross-power spectrum between the interfaces. Figure 5
shows the relative roughness and the phase correlation function for each of the
two interfaces of two SiO2 layers thermally grown on rough silicon.
The behavior displayed by the results shows that the roughnesses of the two
interfaces are highly correlated, that the two interfaces have conformal
long-wavelength roughness, but that at shorter dimensions the growth interface
(in this case, the buried interface) is smoother than the other interface.
This demonstration of scattering ellipsometry presents numerous opportunities
for industrial applications. It establishes the theory for light scattering
from the roughness of interfaces of dielectric films, and therefore offers the
opportunity for the development of light scattering tools that are more
sensitive to yield-reducing defects and particulate contaminants on filmed
materials. The light scattered by roughness of a conformal film is polarized,
which enables the signal from that source to be virtually eliminated, much as
one would use polarizing sunglasses to reduce glare when driving. This new
technique also allows one to perform in situ and non-destructive
characterization of the growth of films on substrates. The light scattering
ellipsometry technique therefore enables the development of improved thin films
used by the semiconductor, optics, visual-display, and data-storage industries.
(T. Germer) |
|
Figure 5. The phase correlation function (top) and the relative
roughness (bottom) of each of the two interfaces of 10.3 nm and 52 nm
SiO2 layers thermally grown on silicon, as measured by light
scattering ellipsometry using 532 nm laser light. Measurements performed
at other wavelengths produced indistinguishable results. |
- Damage Study of Semiconductor Detectors towards 157 nm Excimer
Irradiation. The stability of semiconductor diodes under irradiation from
an excimer laser operating at 157 nm has been evaluated. The Division
built a facility at SURF III that allows simultaneous exposure of
photodiodes to excimer radiation and synchrotron radiation. Measurements of the
spectral responsivity can be made in the spectral range from 120 nm to
320 nm with a standard uncertainty of 0.5 %. The intense, pulsed
laser radiation was used to expose the photodiodes to varying amounts of
accumulated irradiation, whereas the low intensity, broad-band radiation from
the synchrotron source was used to characterize the photodiodes. The changes in
the spectral responsivity of different kinds of diodes, such as UV silicon,
GaP, GaAsP, PtSi, diamond, and GaN, were measured for a large range of total
accumulated dose from an F2 excimer laser operating at 157 nm.
The excimer-laser-induced changes varied by diode and total irradiation dose.
Moreover, it was discovered that the change in diode responsivity was dependent
on the wavelength used to make the measurement. This data yields important
information about the mechanism responsible for the degradation of photodiodes
and about the suitability of the various kinds of detectors for applications in
semiconductor lithography, micromachining, and medicine. (R. Gupta and
P.S. Shaw)
[D]
Figure 6. Radiation damage by a 157 nm laser.
- Vacuum Ultraviolet Irradiance Probe. Many industrial processes, such
as UV curing, photolithography, and semiconductor-chip fabrication, require
accurate measurement of the UV irradiance. In response to this need,
researchers in the Division have constructed and characterized a probe that is
suitable for accurate measurements of irradiance in the UV from 157 nm to
325 nm, where many of the industrial needs lie. The probe consists of a
PtSi detector mounted behind a precision 5 mm aperture. UV synchrotron
radiation from SURF III was used to identify stable detectors necessary
for accurate irradiance measurements and to calibrate the probe. The
measurement of the irradiance is made by scanning the probe in a light field
and measuring the spectral responsivity on a regularly spaced grid. Measurement
of the spectral responsivity in the center of the probe, along with the
integrated total responsivity, yields the spectral irradiance. This method can
alternatively be used to determine aperture areas by taking the ratio of these
two responsivities for an unknown aperture. (R. Gupta and P.S. Shaw)
- New Optical Measurement Technique for the Calibration of CCD Cameras.
Digital cameras are used in a wide variety of applications, including
astronomical observations, geological and earth-resource management, machine
and inspection processes, quantitative microscopy, and video and still
photography. A common type of detector used in digital cameras is the
charge-coupled device (CCD). It contains a two-dimensional array of
lithographically fabricated detectors on a silicon wafer. The number of
individual detector elements can range up to several million.
Division staff recently calibrated a commercial photometric digital camera used
to measure the performance of lamps and LEDs, as well as to quantify
material-appearance characteristics such as reflectance and haze. The
calibration of CCD cameras is difficult since the responses of individual
detector elements can vary, and they are often non-linear. Utilizing the unique
capabilities of the facility for Spectral Irradiance and Radiance Responsivity
Calibrations using Uniform Sources (SIRCUS), we have simultaneously calibrated
the more than 300,000 individual elements in the camera's 640 by 480 pixel
CCD array. The camera's pixel-to-pixel uniformity, non-linearity, and absolute
spectral responsivity were measured on SIRCUS over the visible spectral region
with a relative combined standard uncertainty of 3 %. Commercial cameras
that are calibrated against broadband incandescent sources typically have
uncertainties on the order of 5 %. (S. Brown)
- A Silicon Tunnel-Trap Transfer Standard for Spectral Power and
Irradiance Responsivity Developed. Division staff and Reyer Corporation
have developed a high-accuracy light-trap radiometer. This new six-element
device will be used as a high-level transfer standard to reduce NIST's
measurement uncertainties for spectral irradiance and spectral responsivities.
The heart of the device contains two medium- and four large-size silicon
photodiodes that are tightly packed to obtain a minimum field-of-view of
6°. The device features a precision input aperture whose area was measured
with high accuracy in the Division's Aperture-Area Measurement Facility
(discussed below). The radiometer can be calibrated for spectral power
responsivity against the primary standard High-Accuracy Cryogenic Radiometer
(HACR). The expected combined standard uncertainties of the radiometer in the
power and irradiance measurement modes are 0.03 % and 0.05 %,
respectively. (G. Eppeldauer)
Figure 7. Photodiode arrangement and beam propagation in the triangular
tunnel-trap detector.
- NIST Aperture-Area Measurement Facility Operational. The Division
has commissioned an optical aperture-area measurement facility for apertures
with mean diameters in the range of 3.5 mm to 25 mm. The facility
consists of two instruments. The first is a microscope-based absolute device
utilizing a two-pass, dual-wavelength, laser interferometer. The aperture is
positioned on an air-bearing supported stage, eliminating any hard physical
contact. The instrument is capable of measuring aperture area with total
relative uncertainty of 0.0026 % (k = 2) for round
knife-edge apertures. The second instrument uses flux-transfer techniques to
measure the effective area under specific conditions. Its main purpose is to
calibrate a large group of similar apertures. It is capable of measuring the
effective area to 0.02 % total relative uncertainty
(k = 2) for 25 mm diameter apertures and 0.03 %
total relative uncertainty (k = 2) for 3.5 mm diameter
apertures. The edge quality of an aperture may degrade the uncertainty. A
correction can be calculated that compensates for the degradation.
(J. Fowler)
- Intercomparison of Near-infrared Regular Spectral Reflectance and
Transmittance Measurements. We performed near-normal regular spectral
transmittance and reflectance measurements over the 1 µm to
2.5 µm wavelength region on several different types of materials
using three different spectrophotometer systems in the Division. Two of the
systems employ grating-based monochromators and InGaAs photovoltaic detectors
—the Spectral Tri-Function Automated Reference Reflectometer (STARR), and the
NIST Reference Spectrophotometer for Regular Spectral Transmittance. The third
system uses an FTIR spectrophotometer and a diffuse Au-coated integrating
sphere with a photoconductive HgCdTe detector.
Measurements were performed on transmissive materials as well as highly
reflective or absorptive mirrors and the results were compared, taking into
account any differences in beam geometry and polarization among the different
systems. The results were found to agree within the combined uncertainties.
(L. Hanssen, S. Kaplan, T. Early, and M. Nadal)
- Goniometric System for Variable-angle IR Reflectance, Transmittance,
Polarimetry, and Ellipsometry Measurements. A precision goniometer with
0.01° angular specificity has been coupled to a Fourier-transform infrared
(FTIR) spectrophotometer, allowing absolute reflectance measurements to be made
at angles of incidence from 12° to > 80° and transmittance
measurements to be made from near-normal incidence to 80°. A high-quality
Ge reflective Brewster's angle polarizer with an extinction ratio of
< 10-5 is used to polarize the incident beam. Configured
with a 77 K InSb detector, the system can measure transmittance and
reflectance over the 1 µm to 5 µm wavelength range with an
expanded (k = 2) uncertainty of 0.002. The device is currently
being used to calibrate Au mirrors used by Surface Optics Corporation as
reference standards in their spectral directional hemispherical reflectance
instrumentation. A sample Au mirror reflectance spectrum is shown in
figure 8. This work is being done under the auspices of the Optical
Properties of Materials NIST-Industry Consortium. (S. Kaplan and
L. Hanssen)
[D]
Figure 8. Reflectance of an Au mirror sample at several incident angles
for both s- and p-polarized light, as measured with a recently developed FTIR
based goniospectrophotometer system. The uncertainty in reflectance is 0.002.
- Infrared Transmittance Standards (2 µm to 25 µm).
There has been a need in the optics industry for infrared filters that have
known neutral density across the Short Wave Infrared (SWIR) to Long Wave
Infrared (LWIR) spectral region (2 µm to 25 µm). One of the
main uses of these filters in industry would be for checking the accuracy of
the transmittance scale of infrared spectrophotometers.
|
Using knowledge of the optical properties of metallic films, we designed and
developed multilayer coatings on silicon substrates to meet this need. The
project resulted in high-performance neutral density filters through iterative
computer simulation and experimentation.
The optimized filters are offered as Standard Reference Materials (SRMs).
They consist of Ni:Cr or Cu:Ni films on 25 mm diameter, 250 µm
thick, Si substrates. A thin (20 nm) Si:O coating protects the metal films
against degradation from the air. The SRMs, 2053, 2054, 2055, and 2056, have
optical densities near 1, 2, 3, and 4, respectively. The filters are certified
for near-normal transmittance with a spot size of up to 10 mm diameter,
and a spectral resolution of 8 cm-1. Sample transmittance
spectra for each of the filter sets are shown in figure 9. (S. Kaplan
and L. Hanssen) |
|
 [D]
Figure 9. Transmittance versus wavelength for representative
SRMs 2053 through 2056. The filters consist of thin Ni:Cr or Cu:Ni
coatings on Si substrates. |
- Luminescence from Semiconductor Nanocrystals. Inorganic
semiconductor nanocrystals are currently attracting interest in the development
of nanometer-scale optical probes of biological processes. A novel synthetic
route to produce 2 nm to 6 nm crystallites of these materials has
been developed recently in the Polymer Division, where they employ organic
polymer dendrimers as nanostructure templates. Luminescence from cadmium-sulfide
nanocrystals synthesized via this route is currently being studied in the
Division. In its bulk crystalline form, cadmium sulfide does not exhibit
luminescence because of its indirect energy gap. However, as nanocrystals,
quantum confinement modifies the electronic structure of cadmium sulfide. The
observation of luminescence of various colors confirms such quantum-size
effects. This is illustrated in figure 10, which compares the luminescence
emitted by CdS nanocrystals prepared with different generations of dendrimers
embedded in the organic polymer poly(hydroxyethyl methacrylate). The spectral
peaks display a systematic blue shift in energy as the nanocrystal size is
reduced. The characterization of the structure of this new class of
organic-inorganic hybrid materials by infrared and Raman spectroscopy is
planned. (D.B. Romero, S.W. Brown, and R. Datla)
Figure 10. Luminescence from cadmium sulphide nanocrystals (see
text).
- Raman Spectroscopy of Magnetic Oxides. There is current interest in
materials with 100 % spin polarization at room temperature because of
their potential applications in spin-based electronic devices. The
double-perovskite compound, Sr2FeMoO6, is being actively
investigated for such applications because of its half-metallic nature and high
Curie temperature (TC > 400 K). We have
conducted a detailed investigation of the temperature dependence of the Raman
spectra of this compound to gain insight into the origin of its unusual
magnetic properties. Typical spectra are shown in Fig. 11. We find that a
structural phase-transition occurs at TC, concomitantly with
the paramagnetic to ferrimagnetic phase-transition. Evidence for spin-phonon
coupling is revealed by the temperature dependence of the frequency and
intensity of the apical oxygen stretching vibration around
720 cm-1 being proportional to the square of the magnetization.
These results highlight the importance of spin-lattice interactions in
understanding the properties of Sr2FeMoO6.
Charge ordering in strongly-correlated condensed-matter systems is a topic that
has attracted intense scrutiny because of its possible relevance to diverse
phenomena such as high-temperature superconductivity in the cuprates and
colossal magnetoresistance in the manganites. We elucidate the nature of the
charge-ordered ground state in the manganites by conducting a resonance Raman
investigation of the compound Nd1/2Sr1/2MnO3.
Figure 11. Temperature dependence of the Raman spectra in
Sr2FeMoO6. The insets are crystal structures
corresponding to the spectra (see text).
|
|
Figure 12. Resonance Raman spectra of
Nd1/2Sr1/2MnO3 (see text). |
The main results are shown in figure 12. Resonance enhancement of the
phonons activated by the charge and orbital ordering in this compound is
observed. In particular, we find that the two-phonon scattering peak at
970 cm-1 is resonant with an excitation near 2 eV and a
Brillouin acoustic mode around 25 cm-1 is resonant with an
excitation below 1.5 eV. These results reveal two low-lying excitations of
the charge-ordered ground state in half-filled manganites: a self-trapped
orbital excitation near 2 eV and a possible stripe excitation below
1.5 eV. These findings put severe constraints on theoretical models that
attempt to explain the physics of manganites. (D.B. Romero and
R. Datla)
- BMDO Transfer Radiometer. The Low-Background Infrared Calibration
(LBIR) facility in the Division has recently developed a transfer radiometer
for the Ballistic Missile Defense Organization (BMDO). The BMDO Transfer
Radiometer (BXR) is designed to measure the flux of a collimated source of
light having an angular divergence of less than 1 milliradian. It is
capable of measuring flux levels as low as
10-15 W/cm2 over the spectral range from
2 µm to 30 µm. Spectral resolution is currently provided by
narrow bandpass interference filters and long-wavelength blocking filters. The
radiometer uses an arsenic-doped silicon, blocked-impurity-band (BIB) detector
operated at temperatures < 12 K. All of the components of the
radiometer, which include a mechanical shutter, an internal calibration source,
a long baffle section, a spatial filter, a filter wheel, and a two-axis
detector stage, are cooled to < 20 K with liquid helium. A
cryogenic vacuum chamber has been built to house the radiometer and to provide
mechanical tilt alignment of the radiometer with a source. The radiometer can
be easily transported to a user site. (S. Lorentz)
Figure 13. The BMDO Transfer Radiometer (BXR).
- Calibration Support for NOAA/GOES. A new program was started in the
Division for providing radiometric calibration support to NOAA for the
Geostationary Operational Environmental Satellite (GOES). This satellite system
routinely supplies NOAA with imagery and radiometric measurements for weather
and climate forecasting models. The program will consist of two components. In
one component, the NIST Thermal-infrared Transfer Radiometer (TXR) will be
deployed at a GOES instrument calibration chamber to verify the infrared
radiometric scale used to calibrate some of the GOES instruments. In the second
component, NIST will measure the infrared transmittance of a set of filters
that define the spectral content of the GOES signals. (J.P. Rice,
S. Kaplan, and L. Hanssen)
- Realization of the NIST Detector-based Spectral Irradiance Scale.
Since the early 1960s, the spectral irradiance scales at NIST have relied upon
radiometric transfers from the gold freezing-point blackbody (1337.33 K)
to calibrate sources of spectral irradiance. Because the transfer standard of
spectral irradiance, a 1 kW quartz-halogen lamp, has a spectral
distribution that approximates that of a blackbody radiator at 3000 K, the
process of scale realization requires five intermediate steps to increase the
spectral irradiance output above that of the gold freezing-temperature
blackbody.
The NIST detector-based spectral irradiance scale has been established using a
high-temperature blackbody (HTBB) operated at 2950 K, with the temperature
determined using absolute detectors calibrated for spectral irradiance
responsivity. The spectral irradiances are assigned during a single transfer
procedure using a spectroradiometer that compares the HTBB and the lamps'
spectral irradiance. Previously, we have shown the agreement between the
radiance temperatures above the freezing temperature of gold measured using
absolute detectors calibrated based upon the NIST High-Accuracy Cryogenic
Radiometer (HACR) and the temperatures defined by the International Temperature
Scale of 1990. The spectral radiance of the HTBB has been verified to follow
the Planck spectral radiance distribution by comparisons to a
variable-temperature blackbody with a known, high emissivity (> 0.9999).
The use of the detector-based scale is estimated to reduce the expanded
(k = 2) uncertainties in the spectral irradiance scale, by a
factor of two in the visible spectral region (250 nm to 900 nm) and
an increasing factor of two to ten across the infrared spectral region
(1 µm to 2.5 µm). (H. Yoon)
- LED Photometric and Colorimetric Standards. The Division has
initiated a project to investigate and develop measurement methods and
standards for photometry and colorimetry of light-emitting diodes (LEDs). With
the introduction of high-intensity blue LEDs, potential applications of LEDs
have expanded to include color displays, traffic and aviation signals, and
signs. As the number of LED applications increase, with many directly involving
human safety, accurate specifications of LED characteristics are becoming
increasingly important. Currently, large discrepancies (as much as 50 %)
in photometric measurements of LEDs are reported among manufacturers and users.
To improve this situation, NIST is developing standard LEDs and recommended
measurement methods for luminous intensity, total luminous flux, and color
(chromaticity and dominant wavelength). The NIST work is linked to the
standardization efforts by CIE (Commission Internationale de
l'Eacute;clairage) committees (TC 2–45, 46). Presently, only partial
calibration services for LEDs are available; complete calibrations and standards
for photometric, colorimetric, and radiometric properties of LEDs will be
available soon. (Y. Ohno and C. Miller)
- Double-Quantum Collision-Induced Absorption. Collision-induced
absorptions in and by such gases as nitrogen, oxygen, water, and carbon dioxide
are important contributors to atmospheric opacity, but they are often neglected
in atmospheric modeling due to the lack of reliable quantitative data on these
broad, but weak, absorption features. We recently reported quantitative
collision-induced absorption profiles for atmospheric modeling for the
nominally infrared forbidden vibrational fundamentals. For these profiles, the
optical excitation leads to only one of the collision partners becoming
vibrationally excited, allowing, for instance, Ar to collisionally induce
infrared absorption by N2. Here, we discuss our recent investigation
of the collision-induced absorption by nitrogen-carbon-dioxide collision pairs,
in which both collision partners share the optical excitation. Although, such a
process is significantly weaker than the single quantum excitations observed
previously and is not expected to play a significant role in the atmospheric
opacity, it does provide useful data for evaluating the models and
intermolecular force fields used in the prediction of collision-induced
absorption processes. Such data is valuable due to the present poor quality of
such models.
In our study, we characterized the absorption spectrum of the process
CO2 (v3=1) + N2 (v=1)
CO2 (v3=0) + N2 (v=0) at
4680 cm-1. The spectrum was measured using a Fourier-transform
infrared spectrometer with a long optical path length of 84.05 m and high
pressures (between 5 and 10 times atmospheric). These extreme conditions
were necessary to see this intrinsically weak absorption and to mimic the
kilometer path lengths possible in the atmosphere. (The collision-induced
absorption scales as 
CO2 N2, where is the absorption path length and is a density.)
An experimental value for the absorption coefficient of 1.21(5) x 10-4 cm-2
was obtained at STP,
where T = 273.15 K and
P = 101.325 kPa. A significant discrepancy (30 %)
exists between this value and the previous experimental value obtained by
Fahrenfort [Spectrochim Acta 14, 237 (1959)]. An intensity calculation
has been carried out considering the main terms of the multipole-induced dipole
moment. The exchange repulsion and the dispersion contributions have been
neglected, and an isotropic interaction potential has been considered. An
absorption coefficient of
1.4 x 10-4 cm-2
has been obtained at STP, which agrees within 15 % to the experimental
value. (G.T. Fraser and W.J. Lafferty)
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