Laser velocity-selective excitation

Our studies of the effect of velocity-changing collisions in an rf-laser double resonance experiment contribute to a new vista into the role of collisictis in laser spectroscopy of sub-level structures the limitation of the observation time of the active atoms due to narrow-bandwidth optical excitation and simultaneous velocity diffusion can be of importance for a variety of spectroscopic techniques that use a velocity-selective excitation and detection of either sublevel populations or sublevel coherence. On the other hand, the collisional velocity diffusion of sublevel coherence within an optical Doppler distribution can also give rise to new and surprising phenomena as will discussed in the next section. 

Brook and co-workers were able to excite the HC1 molecules to v = 1 state using a chemical laser and showed that HC1 (v = 1) was 100 times more reactive than HC1 (v = 0) on collision with K atom. In a velocity selected beam experiment it was also observed that for the same amount of energy reagent, vibration was ten times more effective than translational in bringing out the reaction... 

We wish to add that there exists a wide variety of literature that considers the opposite case of monochromatic excitation by an infinitely narrow line causing velocity selection, such as [261, 268, 269, 320, 362] and the sources quoted therein. This description has been developed basically in connection with laser theory it refers most often to stabilized single-mode excitation. The intermediate case between monochromatic and broad line excitation is the most complex one, requiring integration over the modal structure of the laser inside the bounds of the absorption contour [28, 231, 243]. 

These experiments are slightly more difficult in that both the 29s and 27d states must be excited. In this case the atoms are allowed to collide for a few microseconds after pulsed laser excitation, and the 29p atoms are detected as the tuning field is slowly swept over many shots of the laser. The important difference from the Na experiment is that the K atoms can be velocity selected. As shown in Fig. 5, there is a slotted disc in front of the oven, and if the disc is turning, a burst of the atoms passes through the slit, and only those within a narrow velocity range are excited by the 5 ns dye laser pulse 200 /xs after the burst of atoms passes through the slit. 

Similar experiments have been carried out using c.w. dye lasers to probe excited states of BaO generated in a flame of Ba with CO2 diluted in Argon (Field 1981). In this case the lasers were both single mode, so that the pump laser excited a velocity-selected population and the second excitation by the probe laser was therefore sub-Doppler. The schemes and X E" were used to probe... 

In this contribution we present two laser spectroscopic methods that use coherent resonance Raman scattering to detect rf-or laser -induced Hertzian coherence phenomena in the gas phase these novel coherent double resonance techniques for optical heterodyne detection of sublevel coherence clearly extend the above mentioned previous methods using incoherent light sources. In the case of Doppler broadened optical transitions new signal features appear as a result of velocity-selective optical excitation caused by the narrow-bandwidth laser. We especially analyze the potential and the limitations of the new detection schemes for the study of collision effects in double resonance spectroscopy. In particular, the effect of collisional velocity changes on the Hertzian resonances will be investigated. 

It is this group of particles that can be selectively excited to the upper state. This possibility forms the basis for the laser control of the velocity distribution of particles at desired quantum levels. 

In saturation spectroscopy, a monochromatic laser beam labels a group of atoms within a narrow range of axial velocities through excitation or optical pumping, and a Doppler-free spectrum of these selected atoms is observed with a second, counterpropagating beam. 

In two-photon spectroscopy it is possible to record Doppler-free spectra without any need for velocity selection by excitation with two counterpropagating laser beams whose first order Doppler shifts cancel. 

In order to probe the population of rotational levels of the desorbed NO, the time delay between the desorption laser and the LIF probe was flxed, and rotational excitation spectra were recorded. Fixed time delays of 9.0 or 3.0 is (corresponding to velocities of 415 and 1250m/s, respectively) were used to selectively interrogate desorbing molecules belonging primarily to either the thermal or non-Boltzmann component of the total desorbed flux. The desorption Hux in the thermal channel, probed at a time delay of 9.0 /is, was fitted well by a single Boltzmann distribution, with Tf = 200 20 K, somewhat lower than T .,. 

In practice this condition may be fulfilled not only in excitation, e.g. by means of a pulsed laser or a continuous dye laser with insufficient frequency selectivity, but also by means of fines from a continuous gas laser working in simultaneous axial mode u>i (multimode) generation regime see Fig. 3.10(a). Let Au>i = u>i+1 — uii = itc/L denote the mode separation in a laser, L being the resonator length. Then, as pointed out in [110, 127, 231], broad line approximation works if Awj is smaller than the width of the Bennet holes r en [268, 320] in the absorption contour see Fig. 3.10(6). The positions of the Bennet holes are determined by the condition ujq — w/ + kv = 0, where luq is the central transition frequency, k is the wave vector and v is the velocity of the absorbing particle. The broad fine approximation is valid if the following conditions are fulfilled (see Fig. 3.10) ... 

Multiphoton Absorption and Ionization. High laser powers can induce the simultaneous absorption of two or more photons that together provide the energy necessary to excite a transition this transition may be one that is forbidden as a single-photon process (8,297). Such absorption can be made Doppler-free by propagating two laser beams of frequency V in opposite directions, so the Doppler shifts cancel and a two-photon transition occurs at 2v for any absorber velocity. The signal is strong because all absorbers contribute, and peak amplitudes are enhanced by AvD /vN, which may represent a 1000-fold improvement in selectivity and sensitivity. 

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