2 Experimental method

The experimental set-up is already described in detail in Wurps et al. (1996).

Figure 1: Overview over the experimental set-up. The reaction volume is in the centre of the microwave Fabry-Perot cavity. The $\rm H_2O$ molecular beam is generated by a pulsed General Valve nozzle. The $\rm H_2O$ molecules are dissociated by a F2 excimer laser at 157 nm. The OH fragments are detected by LIF via the Q1(3) line

Figure 2: The schematic energy level diagram shows the $^2\Pi _{3/2}(J=7/2)$ state in OH. The arrows indicate the induced microwave transitions and the LIF transitions

A heated General Valve pulsed nozzle is used to produce a molecular beam of rotationally cold water which is directed between the plates of a Fabry-Perot microwave cavity. The Fabry-Perot cavity is part of a microwave system that can be tuned to the resonance frequency of the OH $^2\Pi _{3/2}(J=7/2)$ $\Lambda$-doublet transitions $F\rightarrow F\hbox{$^\prime$ }$around 13.4 GHz. The microwaves are produced by a frequency stabilised klystron and are amplified with an HP 83006A microwave amplifier up to a maximum output of +13 dBm.

A part of the $\rm H_2O$ molecules are dissociated by a F₂ excimerlaser in the center of the microwave cavity. The OH fragments are subsequently detected by LIF with an excimer pumped frequency doubled dye laser via the OH ( $^2\Sigma-^2\Pi$) absorption band around 308 nm. The dye laser is tuned to the Q₁(3) line detecting the upper $\Lambda$-doublet of the $^2\Pi _{3/2}(J=7/2)$ state. The LIF signal is imaged onto an intensified CCD-camera and typically averaged over 100 laser shots.

The relative microwave induced population change between the $\Lambda$-doublets results in a change of the LIF signal. The relative population change was measured for the main lines ( $\Delta F=0$) and the satellite lines ( $\Delta F=1$). Figure 2 shows a level diagram of the hyperfine structure for the OH $^2\Pi _{3/2}(J=7/2)$ state with the induced microwave transitions and the LIF detection.