Collisional resonances molecules

Let us consider a laser oscillating at a single frequency (single-mode operation) and gas molecules inside the laser resonator which have absorption transitions at this frequency. Some of the molecules will be pumped by the laser-light into an excited state. If the relaxation processes (spontaneous emission and collisional relaxation) are slower than the excitation rate, the ground state will be partly depleted and the absorption therefore decreases with increasing laser intensity. 

Collisional redistribution of radiation. A system A + B of two atoms /molecules may be excited by absorption of an off-resonant photon, in the far wing of the (collisionally) broadened resonance line of species A. One may then study the radiation that has been redistributed into the resonance line - a process that may be considered the inverse of pressure-broadened emission. Interesting polarization studies provide additional insights into the intermolecular interactions [118, 388]. 

The depopulation cross sections of the Rb nd states of 25 < n < 40 are 1000 A2, which is the same as the cross section of the Rb ns state if the ns —> (n - 3)1,1 > 3 contribution is subtracted. For the Rb nd states the calculated contribution of the scattering of the nd state to nl S 3 and (n—1)1 s 3 states with no change in the rotational state of the CO is <100 A2, so 90% of the cross section is due to the inelastic transitions leading to rotational excitation. Presumably it is because the resonant transfer accounts for 90% of the observed cross section that the structure in the cross section is more visible in the nd cross sections than in the ns cross sections. For both the ns and nd states minimal collisional ionization is observed and calculated in this n range, principally because there are too few CO molecules with energetic enough A/ = -1 rotational transitions. For example, only CO 7 > 18 states can ionize an n = 42 Rydberg state by a A7 = -1 transition, and only 3% of the rotational population distribution is composed of 7 > 18 states. 

The Ar(3P(, Pt) levels are 11.623 and 11.827 eV, respectively, above the ground (1S) level. The lifetimes are 8.4 and 2.0 nsec (33), respectively. The Ar(3P,1 Pj) states are formed by absorption of the Ar resonance lines at 1067 and 1048 A. In the 1 to 100 mtorr concentration range the lifetime of Ar(3P, P() atoms is of the order of 10 /tsec [Hurst et al. (494)], which is 1000 times as long as that of isolated atoms because of imprisonment of resonance radiation. If the ionization potential ofa molecule is below 11.6 eV, it is possible to increase the photoionization yield (sensitize) by adding Ar to the sample. The increase of the ionization yield is caused by collisional energy transfer between Ar(3P, Pi) atoms and the molecule before the excited atoms return to the ground state by resonance emission. Yoshida and Tanaka (1065) have found such an increase in the Ar propane, and Ar-ammonia mixtures when they are excited by an Ar resonance lamp. Boxall et al. (123) have measured quenching rate constants for Ar(3P,) atoms by N2) 02, NO, CO, and H2. They are on the order of the gas kinetic collision rate. 

The cyclotron orbits of thermal energy ions when they first enter the ICR cell are both too small and incoherent to be detected. However, if an excitation pulse is applied at the cyclotron frequency, the resonant ions will absorb energy and be brought into phase with the excitation pulse. They will have a larger orbital radius and the ion packets will orbit coherently. The ions may then be detected as an image current induced in the receiver plates. Additionally, this excitation pulse increases the kinetic energy of the trapped ions to the extent that fragmentation can be collisionally induced by ion—molecule reactions. Alternatively, the excitation pulse may be used to increase the cyclotron radius so that ions are ejected from the ICR cell. 

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