Knowledge Bank

In this contribution the use of the Cavity Ring Down (CRD) technique for the measurement of the direct absorption spectrum of laser-desorbed diphenyl-amine (DPA) molecules in a supersonic expanding jet is demon-strated. The absorption cross section for the first electronic transition in DPA is measured from the saturation of the DPA ion signal in an independent experiment. From these two experiments the number density of DPA molecules that are brought into the jet expansion via the Laser Desorption technique is determined to be $1011cm-3$. Cavity Ring Down (CRD) spectroscopy, first introduced by O'Keefe and Deacon in $19881$, is a very sensitive technique to perform direct absorption measurements using pulsed light $sources2$. The technique is based on the measurement of the rate of absorption rather than the magnitude of absorption of a light pulse confined in an optical cavity with a high Q-factor. In the experiments presented here two high reflectivity mirrors are mounted in a molecular beam spectrometer as to form a cavity with the axis 10 mm from the nozzle and perpendicular to the beam axis. In this spectrometer DPA molecules are desorbed with a laser pulse from a sample substrate positioned directly behind the nozzle. In this way the direct absorption spectrum of DPA on the $S1\leftarrow S0$ transition could be measured. The absolute absorption cross section $\sigma$ for this transition in DPA is measured in a separate experiment in which the DPA ion yield as a function of excitation laser intensity is measured. In this experiment DPA is excited on the first three bands of the $S1\leftarrow S0$ transition with a pulsed tunable dye-laser at 308 nm. The $S1$ state shows fast inter system crossing to triplet states. After 300 ns an ArF excimer laser is used for ionisation out of these triplet states. Both the 1-color and 2-color REMPI-signals are measured while varying the dye-laser intensity. The cross section $\sigma$ is determined from a fit of the experimental data to analytical expressions derived for this 4-level scheme.