Photofragments, Doppler spectroscopy

Fig. 22. Photofragment Doppler spectroscopy of C3HD4 revealing H-loss, and thus cleavage of the central C—H bond, to be the dominant reaction channel. The allene is the preferentially formed product. (From Deyerl et a .145,149)...

The photodissociation cross sections and angular distributions can be obtained experimentally by different methods for example, time of flight (TOF) photofragment translational spectroscopy (Busch, et al., 1969), Doppler analysis (Xu, et al., 1986), and ion imaging. 

Laser-induced fluorescence photofragment spectroscopy studies with state-selective interfragment recoil measurement have not been performed to any appreciable extent. A preliminary kind of experiment has recently been carried out in an MPD case by using the time-of-flight technique under bulk condition and with rather limited spectral resolution. A Doppler spectroscopy study has been carried out in a one-photon dissociation case where electronically excited fragments were produced under bulk condition, and linewidth measurement was made in emission interferomet-rically. A first experiment with tunable LIF detection has been made recently by McDonald et al., who dissociated HN3 at 266 nm and... 

An alternative spectroscopic approach would be to determine the rotational temperature from the intensities in a rotationally resolved vibrational or electronic spectrum. In this case, even if one cannot resolve individual rovihrational transitions, one can still estimate the temperature by simulating the rotational contour of an individual vibronic band [120]. For this, one needs to know the rotational constants of the molecule and the direction of the transition moment however, even rough estimates of these quantities can lead to a reasonable temperature estimate. In measuring either Doppler widths or rotational band contours, the linewidth one obtains may contain a contribution from the finite lifetime of the molecule, determined by its intramolecular vibrational energy redistribution and/or dissociation rate if some type of photofragment spectroscopy is used, and this can make the temperature appear to be higher than it really is. 

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