Laser-spectroscopic measurements

As a scientific tool, ab initio quantum chemistry is not yet as accurate as modem laser spectroscopic measurements, for example. Moreover, it is difficult to estimate the accuracies with which various methods predict bond energies and lengths, excitation energies and the like. In the opinion of tlie author, chemists who... 

Due to the exceptional importance of hydrogen bonds in biology and chemistry the detailed investigation of their structure and dynamics has attracted the attention of several experimental and theoretical groups. Laser spectroscopic measurements and quantum mechanical calculations have led, in recent years, to remarkable progress to vards the detailed understanding of important prototype systems such as (NH3)2 [1], (H20)2 [2], and DNA base pairs [3],... 

Shimono, A., Koda, S. Laser-spectroscopic measurements of uptake coefficients of SO2 on aqueous surfaces. J. Phys. Chem. 100, 10,269-10,276 (1996)... 

There is a number of other new isotope shift investigations. Some are more pertinent to this discussion of atomic nuclear structure studies, such as the new series of laser spectroscopic measurements of nine tin isotopes, presented at this conference. Others are perhaps more of interest to atomic physics. For example, a systematic study has been initiated of the J dependence of the field isotope shift.On the other hand, recent anomalies in the isotope shifts in samarium have been explained by mixing of closely lying levels by the isotope shift operators, in particular the one describing the field effect. 

Keq = Ksx/Kxs> where K s and Ksx are the rates of intersystem crossing that could be measured by picosecond time-resolved laser spectroscopic measurements. 

According to Kramers model, for flat barrier tops associated with predominantly small barriers, the transition from the low- to the high-damping regime is expected to occur in low-density fluids. This expectation is home out by an extensively studied model reaction, the photoisomerization of tran.s-stilbene and similar compounds [70, 71] involving a small energy barrier in the first excited singlet state whose decay after photoexcitation is directly related to the rate coefficient of tran.s-c/.s-photoisomerization and can be conveniently measured by ultrafast laser spectroscopic teclmiques. 

Optical metiiods, in both bulb and beam expermrents, have been employed to detemiine tlie relative populations of individual internal quantum states of products of chemical reactions. Most connnonly, such methods employ a transition to an excited electronic, rather than vibrational, level of tlie molecule. Molecular electronic transitions occur in the visible and ultraviolet, and detection of emission in these spectral regions can be accomplished much more sensitively than in the infrared, where vibrational transitions occur. In addition to their use in the study of collisional reaction dynamics, laser spectroscopic methods have been widely applied for the measurement of temperature and species concentrations in many different kinds of reaction media, including combustion media [31] and atmospheric chemistry [32]. 

The density here refers to the spatial coordinate, i.e. the concentration of the reaction product, and is not to be confused with the D(vx,vy,vz) in previous sections which refers to the center-of-mass velocity space. Laser spectroscopic detection methods in general measure the number of product particles within the detection volume rather than a flux, which is proportional to the reaction rate, emerging from it. Thus, products recoiling at low laboratory velocities will be detected more efficiently than those with higher velocities. The correction for this laboratory velocity-dependent detection efficiency is called a density-to-flux transformation.40 It is a 3D space- and time-resolved problem and is usually treated by a Monte Carlo simulation.41,42... 

Aside from the direct techniques of X-ray or electron diffraction, the major possible routes to knowledge of three-dimensional protein structure are prediction from the amino acid sequence and analysis of spectroscopic measurements such as circular dichroism, laser Raman spectroscopy, and nuclear magnetic resonance. With the large data base now available of known three-dimensional protein structures, all of these approaches are making considerable progress, and it seems possible that within a few years some combination of noncrystallo-graphic techniques may be capable of correctly determining new protein structures. Because the problem is inherently quite difficult, it will undoubtedly be essential to make the best possible use of all hints available from the known structures. 

Harder, J. W., A. Fried, S. Sewell, and B. Henry, Comparison of Tunable Diode Laser and Long-Path Ultraviolet-Visible Spectroscopic Measurements of Ambient Formaldehyde Concentrations during the 1993 OH Photochemistry Experiment, . /. Geophys. Res., 102, 6267-6282 (1997b). 

Tunable laser spectroscopic techniques such as laser-induced fluorescence (LIF) or resonantly enhanced multi-photon ionization (REMPI) are well-established mature fields in gas-phase spectroscopy and dynamics, and their application to gas-surface dynamics parallels their use elsewhere. The advantage of these techniques is that they can provide exceedingly sensitive detection, perhaps more so than mass spectrometers. In addition, they are detectors of individual quantum states and hence can measure nascent internal state population distributions produced via the gas-surface dynamics. The disadvantage of these techniques is that they are not completely general. Only some interesting molecules have spectroscopy amenable to be detected sensitively in this fashion, e.g., H2, N2, NO, CO, etc. Other interesting molecules, e.g. 02, CH4, etc., do not have suitable spectroscopy. However, when applicable, the laser spectroscopic techniques are very powerful. 

In view of the small sample size, the optical aperture used for pressure tuning vibrational spectroscopic measurements is also very small. This does not create serious problems for high pressure Raman spectroscopic measurements since the laser beam can be focused to 30-40 pm. Thus, the optical system employed in a standard Raman scattering experiment can be used and no special optical interface is required for the pressure tuning Raman spectroscopic measurements. 

The kinetics and mechanisms of radical reactions important in combustion chemistry are best studied under conditions in which single reactions can be isolated rather than in flames where there are multiple pathways for formation and disappearance of the radicals. Reactions of C2 are of particular importance since recent laser saturation measurements in our laboratory (1) have shown that C2 a3IIu is present in oxyacetylene flames at concentrations on the order of 1016 molecules/cm3 (approximately 0.1 torr). Although concentrations of ground state C2 in flames are unknown and cannot be measured by the same technique due to spectroscopic constraints, we expect that C2 X3 g populations are at least comparable. Because of these relatively large concentrations the reactions of both species are of considerable importance in combustion chemistry. However, until recently very little was known about these reactions due to the difficulty of producing a clean source of C2 radicals. 

Until 1992, the accuracy of spectroscopic measurements was limited to 1.6 parts in 1010 by the reproducibility of the 12-stabilized HeNe laser at 633 nm which served as an optical frequency standard, and by the unavoidable geometric wave-front errors in wavelength interferometry. To overcome this limitations it was necessary to measure the optical frequency rather than the wavelength. 

We summarize hoe some considerations affecting the potential accuracy of a spectroscopic measurement of the IS ->2S transition. We shall not dwell on the challenging problems of laser stabilization and optical frequency metrology, but only on the atomic considerations. In particular, we shall consider the major sources of line broadening and possible systematic shifts. We discuss below some of the factors which govern the accuracy of IS —>2S spectroscopy in the hydrogen trap. 

Laser-fluorescence measurements [386] of the ground state MX products of the reactions of Y and Sc with F2, Cl2, Br2 and I2 were analysed assuming Boltzmann vibrational and rotational distributions for the MX product, giving TVib values in the range 2150—2850 K and Ttoi values between 1425 and 1700 K for this family of reactions. This indicates a rather low conversion ( 15%) of the reaction energy into internal energy of the MX product, although it should be emphasised that the above results depend on the spectroscopic and thermodynamic data for the metal monohalides which are not very well established. 

The diffusion coefficient may be measured via several experimental techniques. The most prominent ones at present are the direct observation of a diffusion boundary in either a field electron microscope [159, 160] or a photoelectron emission microscope [158] or via laser desorption experiments [161, 162], In the latter case a short laser pulse is used to heat the surface to momentarily desorb the adsorbate from a well defined region of the crystal. Subsequent laser pulses with well defined time delays with respect to the first one, and measurement of the number of particles leaving the surface, allow one to determine the rate of diffusion into the depicted zone. Other methods to determine surface diffusion are spectroscopic measurements which cover the proper time window, for example magnetic resonance-based methods [163, 164]. In favorable cases these methods may even be applied to single crystal surfaces [165],... 

Direct spectroscopic measurements of absorptions could provide substantial and much-needed complimentary information on the properties of BLMs. Difficulties of spectroscopic techniques lie in the extreme thinness of the BLM absorbances of relatively few molecules need to be determined. We have overcome this difficulty by Intracavity Laser Absorption Spectroscopic (ICLAS) measurements. Absorbances in ICLAS are determined as intracavity optical losses (2JI). Sensitivity enhancements originate in the multipass, threshold and mode competition effects. Enhancement factor as high as 106 has be en reported for species whose absorbances are narrow compared to spectral profile of the laser ( 10). The enhancement factor for broad-band absorbers, used in our work, is much smaller. Thus, for BLM-incorporated chlorophyll-a, we observed an enhancement factor of 10 and reported sensitivities for absorbances in the order of lO- (24). 

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