It has been shown recently that molecular gas samples excited with coherent light can display a variety of transient phenomena, similar to those found in nuclear magnetic resonance. This article elucidates how these coherence effects can be used to isolate or unfold molecular collision mechanisms that normally remain hidden within the optical line shape. Elastic collisions, for example, are easily detected here in two-pulse photon echo experiments for a
13CH3F vibration-rotation transition. The echo-decay function which provides a signature for the velocity-changing collision diffusion mechanism, is not just a simple exponential in time but exhibits an $\mathrm{exp}(\ensuremath{-}K{t}^{3})$ contribution for short times and an $\mathrm{exp}(\ensuremath{-}\ensuremath{\Gamma}t)$ decay for long times. This behavior, which is unknown heretofore, contrasts with spin echoes in molecular liquids where Brownian motion leads only to the cubic decay law. An $\mathrm{exp}(\ensuremath{-}K{t}^{3})$ behavior can be understood in terms of a solution of the Fokker-Planck equation which describes the effect of Brownian motion on echo decay. Such a treatment is valid for small phase excursions; in the case of a gas, this implies the Doppler phase factor $k\ensuremath{\Delta}u\ensuremath{\tau}\ensuremath{\ll}1$, where $\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}$ is the propagation vector of light, $\ensuremath{\Delta}u$ is a characteristic velocity jump for a binary collision, and $\ensuremath{\tau}$ is the echo-pulse delay time. When $k\ensuremath{\Delta}u\ensuremath{\tau}\ensuremath{\gg}1$ as in the long-time regime, the Fokker-Planck solution fails. We, therefore, present a new solution to the Boltzmann transport equation using a weak collision model and find agreement with the entire echo time dependence observed. The echo measurements indicate very small changes in longitudinal velocity per
13CH3F-
13CH3F collision, i.e., $\ensuremath{\Delta}u=85$ cm/sec, thereby justifying the weak-collision model. The total elastic collision cross section is 430
A˚2. It follows that elastic collisions lead to velocity thermalization in a time of \ensuremath{\sim}5 sec when the
13CH3F pressure is 1 mTorr. A comparison is also made of the
13CH3F dephasing time ${\ensuremath{\tau}}_{2}$ in a coherent Raman beat decay, which is independent of velocity-changing collisions, with the longitudinal decay time
T1. Here,
T1 represents the molecule-optical interaction time, due largely to jumps in molecular rotation (
J) and orientation (
M) state, and is obtained from a delayed nutation measurement. The fact that the pressure dependent part of ${\ensuremath{\tau}}_{2}={T}_{1}$ shows that
T1 is also independent of velocity diffusion. Futhermore, when a hole is burned in the Doppler distribution, population recovery must be due to inelastic rather than elastic collisions. Optical Carr-Purcell echoes, multiple pulse echoes, provide a direct measure of the
13CH3F transverse dephasing time
T2 without the effect of elastic collisions while being sensitive to "phase interrupting collisions." Again, we find that
T2=T1 so that phase interruptions are negligible. Had such a process dominated the two-pulse echo, an $\mathrm{exp}(\ensuremath{-}\frac{t}{{T}_{2}})$ damping would have been noticed with no $\mathrm{exp}(\ensuremath{-}K{t}^{3})$ contribution. Thus, the present study covers several new aspects of molecular collisions. It represents the first detailed examination of velocity-changing collisions by coherence methods and without the complication of Doppler broadening.
The contrast and steepness of spectral repers, detected by two nonlinear Doppler free techniques -- photon echo in standing waves (PESW) and saturated absorption spectroscopy (SAS) -- are investigated at 0 yields 1 (nu) 3 P(33) A21 SF6 transition for identical experimental conditions (over gas pressure and exciting beams geometry) versus intensity of CO2 laser radiation. S/N advantage of PESW technique as compared to SAS method is demonstrated. The increasing role of slow molecules in generation of photon echo response at elevated time delays between exciting pulses manifests itself experimentally in slowing down the photon echo decay rate.
Coherent transient phenomena can be generated by the abrupt changes of parameters either of the electromagnetic field or of the matter. Comparative analysis of these two approaches, made on the base of experimental research in molecular gases SF6, 15NH3, HFCO and 13CH3F, excited by the radiation of CW CO2 and 13CO2 lasers, is presented. The first approach of photon echo (and its modifications) generation, based on electro-optic pulses formation from the resonant CW exciting radiation, has proven to be successful for intense infrared molecular transitions. The coherent responses are sufficiently powerful to be applied in high resolution spectroscopy, the spectral markers creation ,and molecular collisions investigations. However, this technique is not very effective for the free polarization decay investigations. Another approach based on the Stark switching molecular levels of polar gas, irradiated by CW laser, was demonstrated for free polarization decay and photon echo generation in 13CH3F. Free polarization decay has shown the pronounced frequency shift because of dynamic Stark effect in the CW radiation field. This specific feature of coherent transients, created by Stark switching, limits the high resolution spectroscopy by the low intensity range of exciting radiation. From other viewpoint, it gives unique possibility to investigate coherent transients generated at the dressed molecular levels.
The contrast and steepness of spectral repers, detected by two nonlinear Doppler free techniques -- photon echo in standing waves (PESW) and saturated absorption spectroscopy (SAS) -- are investigated at 0 yields 1 (nu) 3 P(33) A1/2 SF6 transition for identical experimental conditions versus intensity of CO2 laser radiation. S/N advantage of PESW technique as compared to SAS method is demonstrated. The increasing role of slow molecules in generation of photon echo response at elevated time delays between exciting pulses manifests itself experimentally in slowing down the photon echo decay rate.