PHOFEX spectra

An experimental setup identical to that for PHOFEX spectra can be used to measure correlations between two scalar (internal state population, kinetic energy) quantities or between a scalar and a vector quantity (angular distribution of fragment rotation J or velocity v). By holding the PROBE laser frequency fixed and scanning the MONITOR laser through the Doppler pro-... 

Figure 6 Rate constant for the isomerization CH2CO —> CH2 CO in the threshold region. The solid curve is from PHOFEX spectra of and products. All peaks of 0.2 x lO s" and larger are completely reproducible. The points are direct time evolution measurements with 2a error bars [24].

In the measurements of the PHOFEX spectrum, we scanned the VUV wavelength while the photofragment of S(]5) was monitored by exciting the S(3D )-S(lS) transition by UV laser light. Since only the fluorescence emitted from the S(3D]> fragments in the central region of the free-jet expansion was collected, the photoabsorption of ultracold (-5 K) OCS was selectively... 

Figure 1. Comparison of the VUV-PHOFEX spectrum of a part of the 1E+-1E band of the jet-cooled OCS and the corresponding absorption spectrum measured by McCarthy and Vaida [6].

It can also be noticed in Fig. 1 that spectral features for these three peaks are not symmetrical that is, their spectral shape deviates considerably from a simple Lorentzian line shape. Since the rotational contribution in the peak width in the PHOFEX spectrum is -1 cm-1, which is significantly smaller than the observed peak width, these asymmetrical spectral features are regarded as Fano-type profiles, which can appear in a spectrum for quasibound states. 

Scanning the frequency of the dissociation laser and collecting the total OH fluorescence, while the state-selection and probe frequencies are kept fixed on specific transitions, produces a PHOFEX spectrum an example is displayed in the right-hand panel of Fig. 10. The lines correspond to specific resonance states with rotational quantum number J and projection quantum number if = 2 in vibrational state (6,0,0). If the individual lines are broader than the resolution of the laser system, one can determine the width from fitting the spectrum and thus determine the state-specific dissociation rate. If the true linewidth caused by dissociation is smaller than the resolution of the laser system, the rates can be extracted from time-resolved measurements. All three laser frequencies are fixed, and the OH probe laser used to detect a particular state of OH is delayed with respect to the dissociation laser. In this way one can monitor the appearance of the OH products as function of the delay time, in the same way as described above for N02- In contrast to NO2, however, the rate is a state-specific rate rather than an average rate, because of the high selectivity of the overtone... 

Figure 6.8 The k E) curve for the production of triplet CH2 + CO (v = 0,1) from ketene [CH2CO] near the dissociation threshold. The steps show the onset of new dissociation channels (via transition state vibrational levels) as the energy is increased. The lower curve is a photofragment excitation (PHOFEX) spectrum for CO (v = 0,1 = 2) product spectra collected 50 nsec after the pump pulse. Taken with permission from Green et al. (1992).

Figure 7.26 The dissociation of HjCCO (ketene) produces singlet CH2 + CO without a barrier. The spectrum above is an excitation function (PHOFEX spectrum) in which the singlet methylene Is monitored as a function of the photolysis energy. The signal is integrated over the whole dissociation time so that the steps in the PHOFEX spectrum correspond to the opening up of new CO(/) channels. These appear at their thermochemical onsets which indicates the absence of dynamical barriers in this reaction. Taken with permission from Green et al. (1991).

Figure 3 Dynamically biased spectroscopy of a transition state for dissociation, (a) PHOFEX curve calculated from the measured rates for CH2CO —> CH2 + CO assuming that the distribution of CO(J) states is independent of energy, (b) PHOFEX spectrum at the peak of the CO(J) distribution, CO(J = 12). Evidently, the fraction of CO(J = 12) begins to decrease at about 28,500 cm where this curve flattens out. (c) PHOFEX spectrum for the wing of the CO(J) distribution, CO(J = 2). Peaks show the energies of levels with one quantum of CCO bending excitation at the transition state. The delay time of 50 ns for these curves is short compared to the time for dissociation to be complete [11].

Figure 7 PHOFEX spectrum of the lowest rotational state of ortho singlet methylene near the threshold for CH2CO CH2 + CO. The smoother line is the phase-space theory rate constant. The step positions match the rotational energy levels for free CO.

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