Multiphoton resonance ionization bands in I2

Abstract

Multiphoton resonance ionization (MPRI) bands of I2 vapor have been examined in the 360–600 nm region. A band system with T0 =48 426±4 cm−1 and ωe=254±1 cm−1 is assigned to a two-photon resonant intermediate state involved in either three- or four-photon ionization. Polarization measurements establish that the transition is either 1g or 2g←0g. Comparison to the Dalby system (2d), 5146 cm−1 to higher energy allows a more detailed interpretation of the excited states of both systems: 2g (2Π3/2g) nsσg and 1g (2Π1/2g) nsσg, respectively. Excitation into the visible B←X absorption band between 500 and 600 nm leads to a dense set of MPRI lines. An intense system in this region with T0 =35 762±4 cm−1 and ωe=104±1 cm−1 is assigned to a two-photon resonant intermediate state with polarization measurements establishing it as 0g. It is further interpreted as arising from the two-electron promotion 0g→σu, πu→σu. A striking feature of the MPRI spectra of I2 is the near absence of lines corresponding to the normal optical bands and also the absence of many possible multiphoton transitions allowed by selection rules. The cross section of the B→X transition does occur prominently in the spectrum multiplicatively superimposed on the higher resonances, however.