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Imaging studies of the photodissociation of cations. I. Illustrations of the role of nuclear spin
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Image of FIG. 1.
FIG. 1.

Simulation of the origin band of the ; two photon transition used in the REMPI preparation of cations, using a rotational temperature of and a Lorentzian linewidth of [full width at half maximum (FWHM)]. Images reported in this paper were obtained by exciting the , , and transitions [lines (1), (2), and (3), respectively].

Image of FIG. 2.
FIG. 2.

fragment ion images recorded at , following REMPI preparation of cations via the (a) and (b) two photon transitions of . The corresponding fragment speed distribution is shown alongside each image, together with an assignment comb indicating the odd rotational states of the cofragments. Spectroscopic constants for were taken from Ref. 30, and the threshold dissociation energy for process (1) from Ref. 4.

Image of FIG. 3.
FIG. 3.

fragment speed distribution obtained at , following REMPI preparation of cations via the two photon transition of , at a two photon wavenumber, , together with an assignment comb indicating even rotational states of the cofragments, calculated using spectroscopic constants for from Ref. 30. This alignment of the combs implies implies (with an estimated absolute accuracy better than ).

Image of FIG. 4.
FIG. 4.

product speed distributions from photolysis of parent ions at , (b) , and (c) . The solid lines show results obtained for ions prepared using REMPI transition (1), via an ortho-level of , while the dashed lines are for ions prepared via line (2), involving para-levels of . The assignment combs confirm that ortho-levels of dissociate to give ortho- (odd-) product states (solid ticks), whereas para-levels of yield para- products (even-, dashed ticks).

Image of FIG. 5.
FIG. 5.

product speed distributions from photolysis of parent ions prepared using transition (1), resonance enhanced at the two photon level by the (ortho-) level of , at , (b) , and (c) , together with assignment combs indicating the odd states of the cofragments.

Image of FIG. 6.
FIG. 6.

Relevant parts of the and state PESs of , plotted as a function of , the separation between and the center of mass of the unit, and , the H–H separation, and their seam of intersection in this coordinate space (after Ref. 6). Contours are labeled in units of , with defining the minimum of the potential. The coordinates are scaled by and , respectively, to give a uniform mass-weighted representation.

Image of FIG. 7.
FIG. 7.

Relevant parts of the , , and state PESs of , and their respective seams of intersection, plotted in the same coordinate system as used in Fig. 6 (after Ref. 6). Contours are labeled in units of , referenced to the minimum of the potential (defined as having ).

Image of FIG. 8.
FIG. 8.

Predicted rotational state population distributions resulting from photolysis of cations and subsequent dissociation via routes I [panels (a) and (b)] and II [(c) and (d)]. The parent cations are assumed to be formed by REMPI via, respectively, the [ortho, panels (a) and (c)] and [para, (b) and (d)] levels of the state. Three relative population distributions, each normalized to the same total population, are plotted in each panel. These serve to illustrate the comparative insensitivity of the eventual distribution to the particular ionization tensor [ (left), (center), or (right)].

Image of FIG. 9.
FIG. 9.

Predicted product speed distributions arising from photoexcitation of the mix of states formed by REMPI, via the and levels of the resonance enhancing state [i.e., from a mix of ortho- (solid curves) and para- (dashed curves) levels, respectively, as discussed in the text], and subsequent dissociation via (a) route I and (b) route II. These distributions are plotted on the same product speed scale that applies to the experimental images shown in Figs. 2, 4, and 5. Each product state contribution is modeled by a Gaussian function with FWHM. (c) shows the predicted product velocity distributions obtained by summing equal contributions from routes I and II; the former matches well with the corresponding experimental data [Fig. 2(a)].

Image of FIG. 10.
FIG. 10.

Calculated temperature dependence of the ortho-/para-state population ratio (OPR) in thermally equilibrated samples of (diamonds) and (circles). Rotational state term values were calculated using PGOPHER (Ref. 31) with spectroscopic constants for and from Refs. 15 and 30, respectively.


Generic image for table
Table I.

Ab initio geometries and minimum energies (relative to ground state potential minimum, ) of surface crossings involved in key steps in evolution with initial excitation to (after Ref. 6).


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Imaging studies of the photodissociation of H2S+ cations. I. Illustrations of the role of nuclear spin