Efficiency and mechanism of electronic predissociation of B state I2–Ar

Abstract

The isomer of the I2–Ar complex which yields discrete bands in the B←X spectrum is shown, as expected, to be T shaped on the basis of rotational structure observed in the vibronic bands. Precise fluorescence quantum yields for I2–Ar relative to I2 were measured via simultaneous acquisition of absorption and fluorescence excitation spectra in a slit nozzle expansion. These fluorescence quantum yields provide vibrational predissociation efficiencies for B state I2–Ar as a function of vibrational state from v′ of 15 to 26. This is an oscillating function with local maxima at v′ of 16, 22, and 26. For v′=22 and 26, 73%±3% of the complexes undergo vibrational, rather than electronic predissociation. Fluorescence intensities of combination bands with excitation in the van der Waals modes were also found to have oscillating v′ dependencies with patterns nearly identical to that for the bands without van der Waals mode excitations. Thus, these oscillations must arise from the electronic predissociation channel, rather than the vibrational channel. Deconvolution of the lifetime of B state I2–Ar into vibrational and electronic lifetimes indicates that the similar overall lifetimes at v′ of 18 and 21 result from a twofold increase in the electronic lifetime at v′=21, which compensates for a decrease in the vibrational lifetime. Assumption of a smooth v′ dependence for the vibrational lifetime leads to oscillatory predicted overall lifetimes of 35, 77, 82, 51, and 30 ps over the v′ range of 20–24, respectively. Based on symmetry arguments, as well as the observed vibrational predissociation efficiencies, the electronic predissociation of I2–Ar must arise from coupling of the B state to the Πg state. This coupling may also be the dominant channel for collisional quenching of B state I2.

A three-dimensional wave packet study of Ar⋅ ⋅ ⋅I2(B )→Ar + I + I electronic predissociation

“…This corresponds to the actual conditions of the experiments. 13 The final dissociative state is Ar•••I 2 ͓a( 3 ⌸ 1g )͔, which correlates asymptotically to the Ar ϩ I( 2 P 3/2 )ϩ I( 2 P 3/2 ) limit. Between the B( 3 ⌸ 0 ϩ u ) and the a( 3 ⌸ 1g ) states of I 2 , there is an electrostatic coupling induced by the presence of the argon atom, which is maximum for the T-shape configuration.…”

“…13 Finally, for the coupling matrix element V c between the B( 3 ⌸ 0 ϩ u ) and a( 3 ⌸ 1g ) electronic states induced by the Ar atom, the form that was used in the previous 2D simulations 19 was taken as depending only on the distance between Ar and the center of mass of I 2…”

“…11 For Ar•••I 2 the relative importance of vibrational and electronic predissociation depends very much on the vibrational quantum number vЈ of the initially excited state. 10,11,13 For low vibrational levels (vЈϽ15) electronic predissociation prevails, but for highly excited levels (vЈϾ30) vibrational predissociation dominates. In the range 15ϽvЈϽ30, where the two channels compete, it was found 10,13 that the electronic predissociation rates oscillate as a function of vЈ, and that these oscillations are similar to those observed in the electric-field-induced quenching of the B state of isolated I 2 .…”

“…10,11,13 For low vibrational levels (vЈϽ15) electronic predissociation prevails, but for highly excited levels (vЈϾ30) vibrational predissociation dominates. In the range 15ϽvЈϽ30, where the two channels compete, it was found 10,13 that the electronic predissociation rates oscillate as a function of vЈ, and that these oscillations are similar to those observed in the electric-field-induced quenching of the B state of isolated I 2 . 15 This suggests that the repulsive potential energy surface responsible for the complex-induced electronic predissociation, channel 2, is the same as the one involved in the electric-field-induced predissociation.…”

“…[10][11][12][13][14] When the van der Waals molecule M•••I 2 , where M is a rare gas atom or a molecule, is excited in the spectral region of the I 2 (B←X) transition, the fluorescence excitation spectrum shows broadened features which are attributed to quasibound levels associated to M Since channel 1 produces electronically excited I 2 fragments which can fluoresce while channel 2 is dark, measurements of the M•••I 2 absorption/I 2 fluorescence quantum yield ratio in conjunction with absorption spectra can provide the relative importance of vibrational and electronic predissociation. 13 For He•••I 2 and Ne•••I 2 , vibrational predissociation is much faster than electronic predissociation. For Kr•••I 2 on the other hand, electronic predissociation seems to dominate because fluorescence from excitation of the discrete B←X bands seen in absorption is completely quenched.…”