Velocity Map Imaging Analysis of Manganese Pentacarbonyl Bromide Photodissociation in the Gas Phase and on a Surface

We employ velocity map imaging (VMI) to investigate the photodissociation of manganese pentacarbonyl bromide (MnBr(CO)₅)), both in the gas phase and on a surface. Pulsed 266 nm and/or 230 nm laser light was used to initiate the photodissociation cleaving the Mn-CO bonds. A single 230 or 266 nm photon has enough energy to lead to the dissociation of all five Mn-CO bonds. The CO is ionized using a (2+1) resonance-enhanced multiphoton ionization (REMPI) scheme via the B¹Σ⁺-X¹Σ⁺ transition with ∼230 nm photons. To deconvolute the contributions from the gas-phase and surface photodissociation, one- and two-color experiments are compared. In the one-color experiment, a focused 230 nm light above the sample surface was used to dissociate gas-phase MnBr(CO)₅ and to ionize CO via REMPI. The two-color experiments use grazing angle illumination with a 266 nm laser beam to induce the dissociation of MnBr(CO)₅ on the sample surface and probe with a focused 230 nm laser beam above the sample surface to ionize the product CO at a specific time delay. The CO from the one-color gas-phase photodissociation is seen as a background to the surface dissociation CO signal and was subtracted from both the VMI images and <i>Q</i>-branch REMPI spectra of CO. In the two-color time-resolved detection of CO above the surface after photodissociation at the surface, we observe a complex arrival time distribution. To explain this arrival time distribution, we hypothesize that there are four main pathways for the formation of CO: (1) hyperthermal CO ejected directly into the gas phase from the surface as a result of 266 nm photodissociation, (2) thermalized CO coming from the photodissociation of MnBr(CO)₅ that resides on the surface for sufficient time to come into thermal equilibrium with the surface before desorption, (3) CO from photodissociation of intact MnBr(CO)₅ ejected from the surface and dissociated by the 230 nm laser beam, and (4) CO from the gas-phase photodissociation of MnBr(CO)₅ with 266 nm laser light near the surface. <i>Q</i>-branch REMPI spectra of CO and VMI images were collected at various time delays and are consistent with our hypothesis.