High resolution ion imaging studies of the photodissociation of the Br2+ cation

Abstract

We demonstrate the potential of velocity map ion imaging methods for high resolution studies of the photofragmentation of a state selected molecular ion, Br 2 + . 79 Br 2 + cations were prepared in the v + ¼ 1 levels of both spin-orbit components of the X 2 P g ground state by 2 + 1 resonance enhanced multiphoton ionisation (REMPI) of jet-cooled, isotopomer-selected 79 Br 2 molecules at an excitation wavelength of 263.012 nm. The subsequent photolysis of these state selected ions was then investigated in the wavelength range 372 < l phot < 432 nm by imaging the resulting 79 Br + ions. Use of a home-built Pockels cell assembly allowed measurement of images with and without l phot present, on successive laser shots. The resulting difference image allows clear visualisation of the parent ion fragmentation, free from unintended fragment ion signal induced by the REMPI preparation step. The images reveal participation of dissociation channels yielding Br + ions in both their ground ( 3 P 2 ) and first excited ( 3 P 1 ) spin-orbit states, together with ground state Br atoms. No fragmentation channels involving formation of spin-orbit excited Br( 2 P 1/2 ) atoms are found to be active in this range of photolysis wavelengths. Image analysis yields a precise value for the bond strength of the Br 2 + cation, in both spin-orbit components of its ground electronic state: D 0 [ 79 Br 2 + (X 2 P 1/2,g )] ¼ 23528.1 AE 0.6 cm À1 and D 0 [ 79 Br 2 + (X 2 P 3/2,g )] ¼ 26345 AE 2 cm À1 , and thus a refined measure of the spin-orbit splitting in the ground state Br 2 + cation: A ¼ 2817 AE 3 cm À1 .

Coherent ultrafast photoelectron spectroscopy in Br2: An alternative method for measuring Δv≠ transitions in Rydberg-to-Rydberg excitations

“…Since no information is available for the potentials of either the intermediate or final state, the first approximation we make is that the initial and final state potentials are very similar to the Br 2 + ͑ 2 ⌸ 1/2 ͒ Morse potential. 18,19 We assume that, of the two potentials, i.e., the 4d intermediate and n =8 or n = 9 final Rydberg states, the final Rydberg states will have a potential that matches more closely to Br 2 + ͑ 2 ⌸ 1/2 ͒ and hence we introduce a variable shift to the intermediate 4d-Rydberg state. The extent of this shift is obtained by fitting theoretical time-resolved plots ͑see below͒ to the corresponding experimental phase-shift data for a variety of potentials with shifted internuclear separations for the intermediate 4d-Rydberg state.…”

Collisional trap losses of cold magnetically trapped Br atoms

“…[Ar](t ) vσ dt . (10) The time-dependent Ar density is represented analytically by the amplitude-normalized double-sigmoid function, multiplied by the peak beam number density. This equation is fitted to the data by a least-squares minimization of the peak argon beam density, and a scaling factor that accounts for the fraction of Br atoms produced with trappable velocities.…”

“…This value is in good agreement with that determined experimentally in this work, especially taking into consideration that the orifice of the pulsed nozzle is not fully opened by the retraction of the sealing poppet, and therefore the effective nozzle diameter is slightly smaller than the actual one. Evaluating the integral in equation (10) from the optimum photodissociation delay t D = 775 µs to the end of the pulse gives that 60% of initially trapped atoms are removed by collision with the carrier gas under the experimental conditions used here.…”

“…The decay of the trapped Br atoms is mapped by delaying the probe laser with respect to the gas pulse [6]. Ionization of the Br 2 parent molecules does not contribute to the recorded signal, as the wavelength used in the REMPI detection was far from resonance with the 2 + 1 REMPI transition of Br 2 at 263 nm [10]. The photodissociation process produces ground state Br atoms with a velocity distribution centered on zero.…”

Production of cold bromine atoms at zero mean velocity by photodissociation

“…The position determined agrees with that of the centre of a thermal distribution of Br 2 molecules leaked into the reaction chamber, detected by non-resonant ionization. The velocity of the molecular beam v Br 2 was determined to be 385 ms −1 , by taking an ion image of the Br 2 molecules in the beam, seen as a small spot on the detector following (2 + 1) REMPI at 263.012 nm 24 . From this, the FWHM of the velocity distributions are determined to be 45 and 9 ms −1 for the z-and x-axis velocities respectively, corresponding to parallel and perpendicular beam temperatures of 15 K and 600 mK.…”