State-changing Collisions Research Articles

Sub-Doppler double-resonance spectroscopy of NH_3 using a tunable-diode laser and a CO laser

Results are given for a Stark-tuned double-resonance experiment, using a CO-laser pump and a diode-laser probe. The CO laser, operating on the 13-12 P(15) line at 1775.2588 cm(-1), is locked by Lamb-dip stabilization to one of the Stark components of the a(R)R(9, 9) nu(4) line of NH(3). The diode laser probes the aQ(9, 9) nu(2) line at 921.255 cm(-1), revealing a complex spectrum of sub-Doppler features, the narrowest of which are 5.3 +/- 0.3 MHz wide. As well as the resonances associated with population depletion of the common lower levels, we see line-narrowing effects that are due to two-quantum Raman-type processes and collision-induced resonances arising from state-changing collisions that preserve the molecular velocities. The zero-field a(R)R(9, 9) nu(4) line is established to be 264.1 +/- 5.0 MHz above the CO-laser line.

Collisional narrowing of HF fundamental band spectral lines by neon and argon

Collisional narrowing is observed for high J transitions in the fundamental absorption band of HF in the presence of neon and argon buffer gases, with neon providing the most pronounced Doppler width reduction. Detailed line profiles are measured using a high-resolution tunable laser difference-frequency spectrometer in order to test various collisional lineshape models. The measured lineshapes and the pressure dependence of the widths are least-squares fit in the limits of strong and weak velocity-changing collisions. Though both limits qualitatively explain the data, the systematic discrepancies indicate that an intermediate collision model would be more appropriate. In the case of HF/argon collisions, the resulting line profiles have a slight, but definite, asymmetry, implying a correlation between the velocity- and state-changing collisions.

Effects of velocity-changing collisions on two-photon and stepwise-absorption spectroscopic line shapes

We report the results of an experimental study of the effects of velocity-changing collisions on two-photon and stepwise-absorption line shapes. Excitation spectra for the $3{S}_{\frac{1}{2}}\ensuremath{\rightarrow}3{P}_{\frac{1}{2}}\ensuremath{\rightarrow}4{D}_{\frac{1}{2}}$ transitions of sodium atoms undergoing collisions with foreign gas perturbers are obtained. These spectra are obtained with two cw dye lasers. One laser, the pump laser, is tuned 1.6 GHz below the $3{S}_{\frac{1}{2}}\ensuremath{\rightarrow}3{P}_{\frac{1}{2}}$ transition frequency and excites a nonthermal longitudinal velocity distribution of excited $3{P}_{\frac{1}{2}}$ atoms in the vapor. Absorption of the second (probe) laser is used to monitor the steady-state excited-state distribution which is a result of collisions with rare gas atoms. The spectra are obtained for various pressures of He, Ne, and Kr gases and are fit to a theoretical model which utilizes either the phenomenological Keilson-St\"orer or the classical hardsphere collision kernel. The theoretical model includes the effects of collisionally aided excitation of the $3{P}_{\frac{1}{2}}$ state as well as effects due to fine-structure state-changing collisions. Although both kernels are found to predict line shapes which are in reasonable agreement with the experimental results, the hard-sphere kernel is found superior as it gives a better description of the effects of large-angle scattering for heavy perturbers. Neither kernel provides a fully adequate description over the entire line profile. The experimental data is used to extract effective hard-sphere collision cross sections for collisions between sodium $3{P}_{\frac{1}{2}}$ atoms and helium, neon, and krypton perturbers.

Collisions of xenon (n f) Rydberg atoms with ammonia

In thermal collisions between Xe(nf) Rydberg atoms and NH3 collisional depopulation of the nf state occurs through at least three distinct mechanisms: n-changing collisions, l-changing collisions, and ionizing collisions. Using selective field ionization we have measured the total collisional depopulation rate constants kd for laser-excited Xe(nf) atoms in the n range of 22 to 39. The magnitudes of kd are large, corresponding to reaction cross sections comparable to the geometric size of the Xe(nf) atom at n∼22. In the n range of 25 to 40, Xe+ ion production provides an absolute measure of the rate constants for collisional ionization. Approximate rate constants for n- and l-changing collisions are also presented for the 31f state. In these experiments we find that the state-changing collision rate constants are fairly independent of n and are determined largely by the l-changing collisions. The collisional ionization rate constants measured are smaller than the state-changing rate constants and increase with increasing n, ranging between 0.2 and 4.6×10−7 cm3/sec for n between 25 and 40. The present results are compared to those of recent theoretical calculations.

Study of collisional redistribution using two-photon absorption with a nearly-resonant intermediate state

The authors report studies of collision-induced features of the $3{S}_{\frac{1}{2}}\ensuremath{\rightarrow}3{P}_{\frac{1}{2}}\ensuremath{\rightarrow}4{D}_{\frac{3}{2}}$ two-photon absorption in atomic-sodium vapor undergoing collisions with several rare-gas perturbers. The results yield the collisional redistribution function which describes the nonresonant excitation of the $3{P}_{\frac{1}{2}}$ intermediate state. Good agreement with theory is obtained only if effects of $3{P}_{\frac{1}{2}}\ensuremath{\leftrightarrow}3{P}_{\frac{3}{2}}$ state-changing collisions are included in the calculations. Results are used to obtain a value for a pressure-broadening coefficient in the Na-Kr system.

State-changing collision of a high-Rydberg atom with a polar molecule

Cross sections for $n$- (and $l$-) changing collisions of a high-Rydberg atom with a polar molecule are evaluated and found to be of the order of ${10}^{\ensuremath{-}14}$-${10}^{\ensuremath{-}11}$ ${\mathrm{cm}}^{2}$. This is based on the model of the quasifree behavior of an excited Rydberg electron, with its interaction with the polar molecule playing a dominant role, and on the mechanism due to energy transfer from molecular rotation to the excited electron. It is shown that the magnitude of the cross sections drastically increases as the energy defect for the collision process becomes small. The calculated results for a symmetric-top as well as a linear polar molecule are in reasonable agreement with recent experimental findings. The possibility of learning about rotational deexcitation of the polar molecule by an extremely slow electron impact from experimental information on these processes is briefly discussed.

Molecular Reaction Dynamics

1. Understanding chemical reactions at the molecular level 2. Molecular collisions 3. Introduction to reactive molecular collisions 4. Scattering as a probe of collision dynamics 5. Introduction to polyatomic dynamics 6. Structural considerations in the calculation of reaction rates 7. Photoselective chemistry: access to the transition state region 8. Chemistry in real time 9. State-changing collisions: molecular energy transfer 10. Stereodynamics 11. Dynamics in the condensed phase 12. Dynamics of gas-surface interactions and reactions.