High-resolution Molecular Spectroscopy Research Articles

High Resolution Infrared Studies of Molecular Dynamics

Reaction dynamics is one of the principal subjects of chemistry. The traditional way of studying this problem relies on inspection of changes in concentration of reactants and products as the reaction proceeds; the result of chemical analysis is employed to infer the details of intermediate processes. Chemical reactions are obviously not as simple as explained merely by means of classical methods of chemical analysis. Many workers have closely examined the individual steps of gas phase reaction processes, i.e. elementary reactions. Such efforts have resulted in the detection of quite a large number of short-lived molecules as reaction intermediates. Identification of these species is a sort of prerequisite for understanding the reaction mechanism. Because most reaction intermediates involve un­ paired electrons, they have attracted a lot of attention in many fields of molecular science. Increasing development of spectroscopic tools has enabled workers to characterize many of them. The high-resolution spectra of reaction intermediates are extremely valuable in examining, in real time, distributions over quantum states of molecules taking part in reac­ tions, which can deviate considerably from the Boltzmann distribution. High resolution molecular spectroscopy and kinetic studies have thus collaborated closely to bring about recent remarkable progress in the study of reaction dynamics. Electronic spectroscopy in the visible and ultraviolet regions, combined with flash photolysis, has been most widely employed in the study of transient molecules as reaction intermediates (1). The development of the dye laser has accelerated this trend, because this light source has made laser-induced fluorescence (UF) easy to apply. This method provides us

Picosecond coherent anti-Stokes Raman spectroscopy of molecules in free jet expansions

By using time-resolved coherent Raman scattering, closely spaced vibrational resonances are simultaneously excited and subsequently monitored over long, nanosecond time intervals. Coherent superposition of molecular transitions leads to dramatic oscillations of the picosecond anti-Stokes scattering signals. Experimental data are presented for the nu(1) mode of CH(4) in supersonic expansions at low rotational temperatures (T(ROT) >/= 25). Our timedomain data are in good agreement with spectroscopic results. The corresponding frequency resolution of less, similar3 x10(-3) cm(-1) suggests that this method of Fourier-transform Raman spectroscopy may prove useful in high-resolution molecular spectroscopy.

High resolution molecular spectroscopy in the submillimeter region

Fourier Transform laboratory measurements have been carried out, for the first time in the 8–85 cm−1 spectral region, with an unapodized resolution of 3.3. 10−3 cm−1 and a frequency accuracy of 2. 10−4 cm−1. Samples from spectra of several molecules namely: CO, O3, H2O2, NO, NO2, HNO3, SO2, H2S, HOCL, NOCL, HNCO, ND3 and AsH3 are presented to show both the quality of the measurements and the type of information supplied by high resolution spectroscopy in the submillimeter region.

A tunable, single frequency UV source for high resolution spectroscopy in the 293–330 nm range

An easily tunable, single frequency source of UV radiation is described. The method is based on an intracavity second harmonic generation (SHG) using an angle tuned LiIO 3 crystal in a ring dye laser. The 293–330 nm tuning range with maximum obtainable power of 15 mW and spectral linewidth between 180 and 500 kHz has been obtained for one crystal cut. The method described is simpler, more versatile and covers at least a three times broader spectral range than systems using temperature tuned crystals. Several applications to high resolution molecular spectroscopy are discussed.

Active Raman spectroscopy: high resolution molecular spectroscopical methods

Active Raman spectroscopy, which is based on the coherent excitation of Raman-active molecular vibrations in the field of two strong laser beams is reviewed in respect to its applicability to high resolution molecular spectroscopy. Examples for high resolution CARS, Raman gain, and inverse Raman spectra are given.

Extension of high resolution beam maser spectroscopy into the submillimetre wave region

High resolution molecular beam maser spectroscopy has been extended into the submillimetre wave region of the spectrum. The maser spectrometer consists of a multiple beam source, stacked quadrupole state selector, and Fabry–Perot cavity. The submillimetre wave stimulating signal is produced by harmonic generation from a millimetre-wave klystron, directly referenced to WWVB. The maser signal is detected in a cryogenic photodetector. Linewidths 50–100 times narrower than the Doppler width and good signal-to-noise ratios were obtained. This system has been used to resolve the small deuterium hyperfine splittings of both the 110 → 101 transition of D2O at 317 GHz and the J = 1 → 0 transition of ND3 at 309 GHz. The relevance of the D2O quadrupole interaction data to the electric field gradient in water is discussed. The spectroscopic constants in kilohertz for D2O in the states 110 and 101 are, respectively, (eQqJ)D = 34.81 ± 0.32, CD = −2.661, and (eQqJ)D = −29.60 ± 0.55, CD = −2.532. The constants for ND3 are, in kilohertz: (eQqJ)N = 816.2 ± 1.1, CN = 2.0 ± 1.5, (eQqJ)D = 19.9 ± 0.8, and CD = −2.2 ± 0.8.

High resolution molecular spectroscopy, Third All Union Symposium, 14–16 September 1976

High resolution molecular spectroscopy, Third All Union Symposium, 14–16 September 1976

High-resolution diode-laser spectroscopy of the 9492 cm^−1 band of ethylene

High-resolution absorption spectra within the central maximum of the ν7 band of C2H4 have been recorded using tunable semiconductor lasers. 169 individual transitions have been resolved in three nonoverlapping frequency regions covering approximately 97% of the spectral range from 947.1 to 951.9 cm−1. The frequency splittings between individual transitions are reproducible to ±0.008 cm−1 or better. The problems of scanning an extended frequency range with a multiplicity of diode lasers arise primarily from limited laser-tuning range and the lack of appropriate techniques for precise absolute frequency calibration. The strengths and weaknesses of diode lasers in high-resolution molecular spectroscopy are assessed.

Small bandgap lasers and their uses in spectroscopy

Tunable infrared diode lasers fabricated from single crystals of Pb 1− x Sn x Te and PbS 1− y Se y have been used for a wide variety of high resolution spectroscopy measurements in the wavelength range from 4 to 11 μm. Typical laser threshold current densities range from 50 to 100 A/cm 2 at 4.2°K and from 1000 to 2000 A/cm 2 at 77°K. CW operation is readily obtained at 4.2°K for diode lasers of these alloys and was also observed for PbS at 110°K. The diode laser cw output is typically 10–100 μ watt with an incremental external quantum efficiency of about 1 × 10 -3 . In a few cases, output powers of several milliwatts have been observed. Diode lasers have been made from Pb 1− x Ge x Te and Pb 1− x Cd x S alloys which show promise of extending the emission range of tunable lead-salt lasers to shorter wavelengths. Tuning of the laser emission frequency has been achieved by varying the diode current and temperature and by applying a magnetic field or pressure to the diode. The tuning rates of individual modes were measured using a Fabry-Pérot interferometer and ranged from 10 to 1000 MHz/mA for current tuning and approximately 1 MHz/Gauss for magnetic field tuning. These diode lasers have been extensively used in high resolution molecular spectroscopy. Absorption measurements have been made in sulfur hexafluoride (SF 6 ), ammonia (NH 3 ), ethylene (C 2 H 4 ), carbon monoxide (CO), and sulfur dioxide (SO 2 ). Several other experiments have been performed such as the observation of lambda-doubling, Zeeman splitting and nuclear hyperfine structure of (NO) and the direct observation of lineshape and of the gain (or loss) of several vibration-rotation lines of a CO gas laser. This paper describes the details of the physical properties of lead salt diode lasers as well as the current and potential uses of these devices.

A Graphical Method for Calculating Intensities of Space-Focused Atomic and Molecular Beams

A simple graphical method for calculating focused beam intensities in an atomic or a molecular beam resonance apparatus using space focusing is described. The method takes into account the finite sizes of the source and detector and the distribution of velocities in the beam. It can be used to determine the effect of stops and other obstacles placed in the beam. In addition to serving as a calculational tool, the method provides physical insight into the focusing process. Using the method, the dependence of focused beam intensity on the length of the C field is investigated for a symmetric apparatus. It is found that as the C field is lengthened, the focused beam intensity first increases, then slowly decreases. The results of numerical calculations are presented for a molecular beam apparatus using quadrupole A and B fields, and a beam of carbon monoxide molecules. The implications for high resolution atomic and molecular beam spectroscopy are discussed.

High resolution molecular photoelectron spectroscopy I. Fine structure in the spectra of hydrogen and oxygen

The processes which set fundamental limits to the minimum line widths observable in photo­electron spectroscopy are outlined. An over-all limit of about 0.002 eV is deduced as the smallest attainable line w idth in ordinary circumstances. A photoelectron spectrometer is de­scribed which uses focusing deflexion in a radial electrostatic field for energy analysis. This instrum ent has energy resolution of 0.015 eV, smaller than the normal width of rota­tional envelopes in spectra of polyatomic molecules. The applications to the study of mole­cular hydrogen and oxygen is described.

The electronic structures of methane, ethane, ethylene and formaldehyde studied by high-resolution molecular photoelectron spectroscopy

We have obtained the photoelectron spectra of the compounds in the title using the 584 Å radiation (hν = 21.21 eV) and a high-resolution electron velocity analyser. The results for methane indicate the presence of only one occupied orbital ( pt 2) 6 in the accessible energy range. There is some indication of vibrational fine structure but no detailed analysis is possible and there is some indication that Jahn—Teller splitting may be important. Similar effects are noted in ethane where four orbital levels are found. In both ethylene and formaldehyde extensive vibrational fine structure is found for most of the ionic states and has been interpreted with the assistance of data for the deuterated compounds in terms of the molecular orbitals indicated by the theoretical calculations of earlier workers.