Effects Of Velocity-changing Collisions Research Articles

Artifact side peaks in saturated absorption spectroscopy caused by inappropriate lock-in amplification

Saturated absorption spectroscopy is a powerful technique that provides valuable information about gas-phase atomic targets, including not only the precise positions of transitions but also the effects of velocity-changing collisions, which manifest as changes in the shape of the peaks. However, using a lock-in amplifier inappropriately can generate artifact side peaks, potentially leading to a misinterpretation of results. To investigate the emergence of these artifacts, we conducted numerical calculations based on the basic Lambert–Beer’s law. Our model reproduced the artifact side peaks and revealed that these artifacts become more significant as the transmittance of the probe laser decreases.

Pressure and temperature dependencies of air-perturbed O[formula omitted] B-band line shapes

The air-broadened lines from the oxygen B band were measured for the first time in controlled laboratory conditions with a high signal-to-noise ratio using frequency-stabilized cavity ring-down spectroscopy (FS-CRDS) referenced to the optical frequency comb. Spectra measured at various pressures and temperatures were analyzed with an advanced line-shape model, considering the speed-dependence of collisional broadening and shift, and the effect of velocity-changing collisions. The temperature dependence of collisional broadening and shift is determined, whereas no significant temperature dependence of speed-dependent parameters and Dicke narrowing was observed. In addition, we have demonstrated that reasonable estimation of temperature dependence for pressure broadening is possible even from measurements done in single temperature where the speed dependence of pressure broadening was determined. New spectroscopic data improve the accuracy of the air-broadened oxygen B-band spectra description by an order of magnitude.

Velocity changing collisions in the laser saturation spectra of 87Rb D 2 F″ = 2

The saturation absorption technique is applied to the 87Rb 2S 1/2 F″ = 2 → 2P 3/2 F′ = 1, 2 and 3 transitions to study the effect of velocity changing collisions (VCC). The VCC caused Doppler pedestal increases with argon pressure from 0 to 110 mTorr and decreases with modulation frequencies of 700–3200 Hz. The resonances of the velocity selective, saturated optical pumping are washed out for pressure of 110 mTorr. The magnitude of the Doppler pedestal relative to the homogeneous features, yields a rate for velocity changing collisions of 6.5 ± 0.2 × 10 −10 cm 3 s −1.

An investigation of collisional processes in a Dicke narrowed transition of water vapor in the 7.8 μm spectral region by frequency down-chirped quantum cascade laser spectroscopy

Information about intermolecular potentials is usually obtained through the analysis of the absorption line shapes recorded in the frequency domain. This approach is also adopted to study the effects of motional narrowing and speed dependence of the pressure broadening coefficients. On the other hand, time domain measurements are directly related to molecular collisions and are therefore frequently employed to study molecular relaxation rates, as well as the effects of velocity changing collisions and the speed dependence of the absorption cross sections. Intrapulse quantum cascade laser spectrometers are able to produce both saturation and molecular alignment of the gas sample. This is due to the rapid sweep of the radiation through the absorption features. In the present work the frequency down-chirped radiation emitted by an intrapulsed quantum cascade laser operating near 7.8 mum is employed to investigate the collisional relaxation processes, and the collisional narrowing, in the 15(0,15)<--16(1,16) and 15(1,15)<--16(0,16) doublet in the water vapor nu(2) band. The effects of He, Ne, Ar, N(2), and CO(2) as collisional partners are investigated. The experimental results clearly indicate the dependence of the collisional cross sections upon the chirp rate. They also demonstrate that by using different chirp rates it is possible to gain information about the intermolecular processes driving the molecular collisions and the related energy transfer.

Laser frequency stabilization using an Fe-Ar hollow cathode discharge cell

Polarization spectroscopy of an Fe-Ar hollow cathode discharge cell was used to lock a frequency-doubled Ti:sapphire laser to the 372-nm5D4→5F5 transition of 56Fe. The discharge cell produced a density of 1018 m-3 ground-state 56Fe atoms at a temperature of 650 K, this density being comparable to a conventional oven at 1500 K. Saturated absorption spectroscopy and two schemes of polarization spectroscopy were compared with respect to signal-to-background ratio and the effect of velocity-changing collisions. The laser was locked within 0.2 MHz for hours by feedback of the dispersive polarization spectroscopy signal.

Diode laser spectroscopic measurement of line shape of (ν 1 +3ν 3 ) band transitions of acetylene

A near-infrared diode laser spectrometer is set up to study the absorption line shape of acetylene in the 782 nm region. The second-derivative spectra recorded by source modulation technique have enhanced sensitivity. Careful choice of operating current and diode temperature leads to distortion-free line shape for six rotational components of the (ν1+3ν3) overtone-combination mode of acetylene. Self- and nitrogen-broadening coefficients and line-strength parameters have been extracted by fitting the observed line shape with Voigt profiles. There is no evidence of the effect of velocity changing collisions on the line shape in this near-infrared band.

Effects of velocity changing collisions on line shapes of HF in Ar

The generalized Hess method (GHM) gives a line shape expression which is formally equivalent to the Rautian–Sobel’man hard collision model of Dicke narrowing, but differs radically in the definition of one of the relaxation terms. The relaxation term leading to pressure broadening is the same, but the term leading to Dicke narrowing and ultimately to Doppler line shapes at zero density differs in certain important respects: (1) in GHM it is a weighted sum of the pressure broadening coefficient and an optical diffusion coefficient and (2) there is no sharp distinction between ‘‘velocity changing’’ and ‘‘phase changing’’ collisions. The Dicke narrowing term should thus be understood as including both collision types irretrievably intermixed, with GHM providing a prescription for both relaxation terms. Applied to HF v=0→1, j→j±1 absorption spectra in a bath of Ar and using an accurate interaction potential obtained from spectra of the van der Waals complex and essentially exact close coupling scattering S matrices, GHM provides a rather good description of recently measured line shapes.

Study of collisional effects on band shapes of the ν1/2ν2 Fermi dyad in CO2 gas with stimulated Raman spectroscopy. II. Simultaneous line mixing and Dicke narrowing in the ν1 band

An experimental (SRS) and theoretical analysis for the ν1 component of the ν1/2ν2 Fermi dyad of CO2 has been performed for densities lying from 0.01 to 50 amagat at 295 K, and from 0.01 to 20 amagat at 500 K. At subatmospheric pressure, both line mixing and Dicke narrowing take place for this component due to the very weak Q line spacings. A simple method to account for both diffusional narrowing (due to velocity changing collisions) and collisional narrowing (due to energy transfers) on isotropic Raman Q-branch profile is proposed. This method is based on the transformation of the collapsed Q-branch profile as a sum of individual Lorentzian plus dispersive components whose parameters are density-dependent. Such an exact transformation permits to easily introduce the averaging effect of velocity changing collisions on each component, and then on the collapsed Q-branch itself. In the present study, the Galatry soft collision model is used to define a generalized complete profile for each Lorentzian plus dispersive component. Such a procedure allows us to take into account the velocity changing collision’s effects not only on isolated lines (the well-known Dicke narrowing) but also on the line couplings resulting from collisionally induced rotational energy transfers. The present analysis permits an accurate description of the observed modifications on the SRS profile of the ν1 band of CO2 (1388 cm−1) as a function of density. The straightforward extension to other spectroscopies (linear and nonlinear) is suggested.

Collisional relaxation in an extended-pulse photon echo: Weak-field limit.

Abstract : The effect of velocity-changing collisions in an extended-pulse photon echo is studied. In an extended-pulse photon, the second excitation pulse has a duration comparable with the relaxation time of an atomic system. An analytic solution of the quantum-mechanical transport equation is presented in the perturbation-theory limit and applied to the extended-pulse photon echo. It is found that the decay rate of the echo signal due to velocity-changing collisions is significantly modified by the variation of the second pulse's duration. The results are compared with those of conventional theories of two- and three-pulse echoes using temporally narrow excitation pulses. A physical interpretation of the results is given using both time- and frequency-domain arguments. Reprints. (JHD)

The influence of velocity-changing collisions on resonant degenerate four-wave mixing

The phase-conjugate signal observed in resonant degenerate four-wave mixing on the 6/sup 3/P/sub 2/ to 7/sup 3/S/sub 1/ transition of atomic mercury in a Hg-Ar discharge is investigated. At a fixed argon pressure the variation of the signal with pump powers is explained by a model that includes the effects of velocity-changing collisions (VCCs). As the argon pressure was varied from 0 to 1 torr an increase in the phase conjugate signal was observed and is ascribed to a change in the discharge dynamics with argon pressure and to the influence of VCCs. To further clarify the role of collisions and optical pumping, degenerate four-wave mixing spectra are examined as a function of pump power. Line shapes are briefly discussed. >

Effects of velocity-changing collisions on nonlinear mixing in resonant systems

We examine the effects of velocity-changing collisions (VCC’s) on nonlinear mixing in a Doppler-broadened vapor. We use a model in which both velocity-changing and phase-interrupting collisions are accounted for. We present explicit calculations and results for four-wave mixing in two-level systems for the case of strong collisions. We give results for both phase-conjugate and forward geometries. We discuss new features and modifications of signal line shapes due to VCC’s. The line shapes reflect the various collisional processes that occur in the system. VCC-induced line narrowing is discussed in detail.

Experimental determination of Na–noble-gas velocity-changing and fine-structure-changing collision kernels

We report on an experimental investigation of the collision kernels governing the effects of velocity-changing collisions, fine-structure-changing collisions, and their interplay, for Na--noble-gas pairs. The velocity distributions in the two Na excited-state fine-structure levels and in the Na ground-state hyperfine levels have been measured in the presence of a pump laser which produces velocity-selective excitation. For interpretation of these experiments we use a rate-equation model developed in the context of light-induced drift: the velocity-changing collisions are accounted for by a composite Keilson-Storer kernel describing large-angle and small-angle collisions and the fine-structure-changing collisions are treated in the sudden limit. The model is valid for arbitrarily high pump laser intensities and uses only a single adjustable parameter characterizing the small-angle scattering. It is found that the theory describes the experimental velocity distributions in all four Na levels over a wide range of experimental parameters for each Na--noble-gas pair. The relevance of the collision kernels deduced in the present work is discussed in the context of light-induced drift.

Hyperfine structure measurements in 51V using Doppler-free saturated absorption and polarization spectroscopy

The hyperfine structure in the six fine structure levels in the 3d 44p 6F term in 51V has been studied and the magnetic dipole interaction constants were determined employing Doppler-free saturation spectroscopy on vanadium atoms produced by sputtering in a hollow-cathode discharge. The drastic improvement that can be obtained in saturated absorption spectroscopy by modulating the pump beam at a high modulation frequency is clearly demonstrated in the present paper. For these high modulation frequencies a comparison with polarization spectroscopy showed that the intrinsic advantage in signal-to-noise ration that polarization spectroscopy has at low modulation frequencies is essentially lost at higher modulation frequencies. Also, in this particular case, the effects of velocity changing collisions were rather small. Consequently the two methods were considered being equally powerful in the present measurement situation.

HIGH RESOLUTION INTERMODULATED AND DOUBLE RESONANCE ATOMIC SPECTROSCOPY IN A HOLLOW CATHODE

The performances of a hollow cathode discharge tube developed for high resolution optogalvanic spectroscopy of both refractory and volatile elements are illustrated. Hyperfine and isotopic structures are resolved. The effect of velocity changing collisions is investigated on different transitions. Doppler-free optical-optical Double Resonance measurements are reported for the first time in optogalvanic detection.

Collisional effects upon optical coherences: Experimental study in a xenon three-level system

Laser induced nonlinear absorption profiles in a three-level system have been recorded to study the effect of elastic collisions upon optical coherences. In our particular system, the signal is due to a pure double-quantum term, where we have separated the effect of phase-interrupting collisions from the effect of velocity-changing collisions. Experimental results show that the effect of velocity-changing collisions is very small, and that broadening of the signal is due to phase-interrupting collisions mainly. We are able to determine the increase of the decay rate of the coherent superposition (the “coherence”) of two levels of same parity [Dγ 13/dp=(6.5±1) MHz/Torr] and an upper limit for the mean velocity change (ũ≲5m/s).

Backward Echo in Two-Level Systems

The possible three-pulse echoes in two-level systems of solids and gases are discussed with particular emphasis on backward-wave echoes. One example of the backward echoes was realized with the atomic sodium $D$ line. The detection of this echo requires neither optical shutters nor magnetic field. The decay rate of the echo due to Na-Ar collisions was measured. It is shown that the effect of velocity changing collisions is negligible compared with that of phase interrupting ones.

A suitable system for studying the effect of velocity-changing collisions in saturated absorption: Xe He

In a two-laser experiment the authors measure the broadening of the saturated absorption profile of the 3.51 mu m line of xenon, in the presence of helium. The pressure of xenon is fixed at 3 mTorr, and the helium pressure is varied between 0 and 550 mTorr. The computation of the profile includes the effect of elastic collisions without any adjustment of parameters. This is made possible since all the necessary parameters are known and a particularly simple description of the collisions is valid (Brownian motion, hard-sphere potential). The comparison with the experimental data shows that there is satisfactory agreement at high pressures of helium but an unexplained discrepancy exists at low pressure which could be attributed to inelastic processes.

Study of velocity-changing collisions in excited Kr using saturation spectroscopy

A saturated-absorption experiment is used to measure the effects of velocity-changing collisions between Kr* metastable atoms (4p/sup 5/5s(3/2)/sub 2/) and He and Ar perturbers. Comparison between experimental profiles and profiles calculated assuming hard-sphere collisions between Kr* and perturber atoms confirms the calculations of Borenstein and Lamb concerning the change of velocity associated with a collision. Furthermore it is found that the rate of velocity-changing collisions is consistent with the predictions of kinetic theory.

The effect of velocity-changing collisions upon saturated-absorption profiles: the laser line of xenon at lambda =3.51μm

Parameters giving the broadening due to Xe perturbers of the laser line of xenon at lambda =3.51 mu m are obtained in linear absorption and compared with those found in saturated absorption. The results are found to differ significantly. Furthermore, the non-linear variation of the broadening constant with pressure observed in saturated absorption does not appear in linear absorption. The data can be explained by including the effects of weak velocity-changing collisions (VCC) upon saturated-absorption profiles. We show that under these conditions the width of the narrow resonance of saturated absorption is due mainly to the effects of weak VCC.

Effect of velocity-changing collisions upon optical coherences in a three-level system

In this work, a theory of laser nonlinear spectroscopy and its application to the study of atomic collisions is developed. A three-level system (TLS) is considered in low-pressure gases. The effect of weak velocity-changing collisions (vcc) upon atomic coherences is introduced into the TLS equations established by H\ansch and Toschek. It appears that a TLS is suitable for the observation of the effect of vcc upon optical coherences.