Laser double resonance technique

In this chapter we shall treat the most important laser double-resonance techniques by illustration with several examples. While the pump transition is always induced by a pulsed or cw laser, the probe field may be provided by any coherent source in the spectral range between the RF region and the ultraviolet. 

Small-scale laboratory-sized chemical lasers provide convenient sources for studies of the distribution of vibrational energy in chemical reaction products and for measurements of vibrational and rotational energy transfer processes with the laser-induced fluorescence and laser double-resonance techniques. " ... 

The YS molecule has been studied by Azuma and Childs [82], again using the molecular beam laser/radioffequency double resonance technique. The electronic spectrum arising from the /i 2 E 1 Y 2 E 1 transition was studied through fluorescence... 

Figure 12-1. Schematic diagram to illustrate double resonance techniques, (a) REMPI 2 photon ionization. The REMPI wavelength is scanned, while a specific ion mass is monitored to obtain a mass dependent SI <- SO excitation spectrum, (b) UV-UV double resonance. One UV laser is scanned and serves as a burn laser, while a second REMPI pulse is fired with a delay of about 100 ns and serves as a probe . The probe wavelength is fixed at the resonance of specific isomer. When the burn laser is tuned to a resonance of the same isomer it depletes the ground state which is recorded as a decrease (or ion dip) in the ion signal from the probe laser, (c) IR-UV double resonance spectroscopy, in which the burn laser is an IR laser. The ion-dip spectrum reflects the ground state IR transitions of the specific isomer that is probed by the REMPI laser, (d) Double resonance spectroscopy can also use laser induced fluorescence as the probe, however that arrangement lacks the mass selection afforded by the REMPI probe...

Monohydrates of water have been investigated experimentally using the same laser techniques as those employed for guanine [17, 43, 44], Again studying methylated species and using UV/UV double resonance techniques have provided evidence for the existence of several monohydrates. Resolved band spectra are observed for the hydrates UV origin transitions of three monohydrates have been found in the... 

One of the important features of SEP, in common with other optical-optical double-resonance techniques, is the simplification of the spectra because laser I excites a single upper state. Thus, selection rules between the intermediate and final states greatly reduce the final allowed states. Of particular interest for the study of dissociation phenomena is that SEP allows for considerable control of the final states that are... 

Volume 11/19 brings the spectroscopic data on diamagnetic and paramagnetic molecules as well as on molecular ions up to date considering the publications up to and partly including 1990. The spectroscopic information collected in this volume has been obtained principally from gas phase microwave measurements. In addition, gas phase data have been included derived from methods related to microwave spectroscopy by employing a coherent radiation source. These are molecular beam techniques, radio frequency spectroscopy, electron resonance spectroscopy, laser spectroscopy, and double resonance techniques. Some other methods are considered if the accuracy of the derived molecular parameters is comparable to that of micro-wave spectroscopy and no microwave data are available. Examples would be Fourier infirared spectroscopy or electric deflection method. 

If the probe laser-induced fluorescence Im(X2) is monitored through a lock-in amplifier at the frequency /i, one obtains negative OODR signals for all transitions 1) -> m) and positive signals for the transitions 2) m) (Fig. 5.17). From the phase of the lock-in signal it is therefore, in principle, possible to decide which of the two possible types of probe transitions is detected. Since this double-resonance technique selectively detects transitions that start from or terminate at levels labeled by the pump lever, it is often called labeling spectroscopy. 

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. 

Recently, a novel rf-laser double resonance method for optical heterodyne detection of sublevel coherence phenomena was introduced. This so-called Raman heterodyne technique relies on a coherent Raman process being stimulated by a resonant rf field and a laser field (see Fig.l(a)). The method has been applied to impurity ion solids for studying nuclear magnetic resonances at low temperature3 5 and to rf resonances in an atomic vapor /, jn this section we briefly review our results on Raman heterodyne detection of rf-induced resonances in the gas phase. As a specific example, we report studies on Zeeman resonances in a J=1 - J =0 transition in atomic samarium vapor in the presence of foreign gas perturbers. 

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. 

Clarke and Hofeldt determined the depopulation rates for the individual triplet state spin sublevels of chlorophyll a and chlorophyll b by microwave-modulated fluorescence intensity measurements. The species was dissolved in n-octane at a temperature of 2 K. The solvent n-octane is a low-temperature host matrix which allows high-resolution spectroscopy in the chlorophyll triplet state. Triplet absorption detection of magnetic resonance as well as fluorescence-microwave double resonance techniques were applied. The experimental arrangement was described in Ref. 167. In the case of fluorescence detection, chlorophyll b was irradiated with the 457.9-nm single-mode line of an Ar" laser. Microwave transitions were... 

The technique to shift molecular transitions into coincidence with fixed frequencies of a laser is called laser magnetic resonance (LMR). This type of spectroscopy, which is used for studying unstable molecules, was combined with the double resonance technique by Lowe and McKellar. A radiofrequency double resonance cell was placed in the 6-cm gap of the electromagnet of an intracavity laser magnetic resonance apparatus. The Zeeman magnetic field was oriented parallel to the laser light polarization (AM = 0 infrared transitions) and perpendicular to the electric field vector... 

Of wide use are now various kinetic spectroscopic methods which permit to follow the time evolution of the intensity of a particular optical or infrared transition. This provides direct measurements of the relaxation rate constants. Also, the double resonance technique should be mentioned. It uses one laser for the population of a particular vibrational molecular level and another (of a much lower intensity and of a quite different frequency) to monitor the population change of this and the neighbouring levels caused by relaxing transitions. 

Vibrational relaxation of the X v" = 1 level of NH and ND in noble-gas matrices was studied by an optical-optical double resonance technique (using excitation of A v = 0 by a pump laser and probing the X v" = 1 population by another laser with variable time delays). The following lifetimes x were obtained at 4 K in photolyzed NH3 (NDgj-noble gas (1 10000) samples [2] ... 

This optical-optical double-resonance technique has already been used for other Doppler-free techniques [10.25], such as polarization spectroscopy (see Sect.10.3). Its applications to molecular beams has, however, the following advantages compared to spectroscopy in gas cells. When the chopped pump laser periodically depletes the level E. and populates level Ej, there are two relaxation mechanisms in gas cells which may transfer the population modulation to other levels. These are collision processes and laser-induced fluorescence (see Fig.8.39). The neighboring levels therefore also show a modulation and the modulated excitation spectrum induced by the probe laser includes all lines which are excited from those levels. If several absorption lines overlap within their Doppler width, the pump laser simultaneously excites several upper states and also partly depletes several lower levels. 

A third method, which comes more and more into use, is based on various optical double resonance techniques and requires generally at least two tunable lasers. The essential point of this method is the labelling of a selected molecular level by optical pumping with a "pump laser". A second "probe laser" is tuned through the spectral range of interest, but only those molecular transitions are monitored, which start from the "labelled" level, selected by the pump transition. The method works with pulsed and with cw lasers. Combined with one of the sub-Doppler techniques a drastic reduction of the line density as well as a reduction of the line width can be achieved. This makes double resonance techniques very attractive. 

Figure 3.19. Illustration of the double-resonance technique. The pump laser operates at line center on the Fi(4) transition to produce HF molecules in the i = 1, / = 3 state. These molecules have a narrow velocity range compared to that associated with the Doppler-broadened line profile. A probe laser on the 2(3) transition, tuned to coincide with the narrow frequency range associated with the velocity class of laser-excited t = 1, / = 3 molecules, may be used to monitor the laser-excited population. The probe laser can also be tuned to monitor the populations of nearby rotational states in the v = level, as shown.

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