Rotational Alignment in Associative Desorption ofD2(v′′=0and1) from Pd(100)

We report the first measurement of the preferential steric orientation of ${\mathrm{D}}_2$ molecules associatively desorbing from a metal surface. The flux of ${\mathrm{D}}_2$ desorbing from Pd(100) is probed by laser induced fluorescence with linearly polarized tunable vacuum ultraviolet radiation in the $B{}_{}{}^1{}_{}{}^{}\Sigma _u^{+}{}_{}{}^{}\left(v^{\prime }{,J}^{\prime }{,M}^{\prime }\right)\leftarrow X{}_{}{}^1{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}\left(v^{\prime \prime }{,J}^{\prime \prime }{,M}^{\prime \prime }\right)$ Lyman bands. In $(v^{\prime \prime }=0\mathrm{})$ an increasing positive alignment with rotational quantum number is observed up to $J^{\prime \prime }=6\mathrm{}$, establishing the preferred helicopter motion of the molecules. In $(v^{\prime \prime }=1\mathrm{})$ and also for ( $v^{\prime \prime }{=0,J}^{\prime \prime }=7\mathrm{}$ and $8$) an isotropic J vector distribution is measured.