Abstract: Ion dip spectroscopy, a folded OODR process with ion detection, has hitherto only been applied to the study of high-lying vibrational levels of ground state polyatomics. We have succeeded in employing this technique to obtain rotationally resolved spectra of NH_3Ã_1A_2″, a fast predissociating species. Conceivably, the method can be generalized to deal with a large number of other predissociating species.
We first obtain high resolution REMPI spectra of C′←X transition via a 3+1 photon scheme by scanning a pulsed dye laser in the 430—480 nm. A sample spectrum of the 0-0 band is shown in Fig.1. Rotational constants B_0=10.80±0.01 cm~(-1) and C_0=5.23±0.01 cm~(-1) are obtained for the C′~A_1′v_2=0 level predissociation lifetimes are inferred from the rotational linewidth for the C′v_2′=0,1,2 levels. We then induce the C′→Ã stimulated emission by scanning a second dye laser from 560—580 nm while holding the first laser to a specific C′←X rovibronic transition. The competition between C′→A and the ionization process from C′ state results in the ion dip spectra shown in Fig.2. The rotational lines of C′→A 0-0, 1-1 and 2-2 bands are listed in Table 3. While selection rules allow Δv_2=0,±2,±4... transition, Franck-Condon factor favors Δv_2=0 only.
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