Paper SS2-WeA11
3-Dimensional Velocity Map Imaging from Surfaces: A New Technique for the Study of Photodesorption Dynamics

Wednesday, November 11, 2009, 5:20 pm, Room N

The ion imaging technique and the improved velocity map imaging (VMI) technique [1] have been successfully applied to many gas-phase photo-dissociation and crossed-beam experiments over the last 20 years [2]. The VMI technique maps two dimensional product velocity distributions to an image on a position sensitive detector and thus allows the measurement of product flux as a function of velocity for all velocities simultaneously.

Experiments on photo-desorption from surfaces have so far not used this elegant technique. Such experiments have often relied on time-of-flight (TOF) techniques to gain dynamical information about the process under investigation [3].

Here we show how Time-of-Flight (TOF) measurements can be coupled to the VMI technique to yield 3-dimensional velocity distributions for desorption products of surface photochemistry. In one example, we applied this Three-Dimensional Surface Velocity Map Imaging (TDS-VMI) method to measure 193 nm photo-desorption of Br atoms from a KBr surface. Although this system has been studied in the past, the TDS-VMI technique reveals new features of the desorption dynamics. The velocity distributions indicate that at least two non-thermal mechanisms contribute to the photo-desorption dynamics. The TDS-VMI method also allows us to measure the yield of Br(²P_3/2) and Br(²P_1/2) over the full range of velocities that contribute to the photo-desorption process. The branching ratio Br(²P_3/2)/Br(²P_1/2) is 24/1, a value that is quite different from that obtained previously.

Other examples will be discussed at the conference.