Apparatus for the study of electronic spectra of collisionally cooled cations: para-dichlorobenzene

An apparatus has been developed with the aim of measuring the electronic spectra of large ions at low vibrational and rotational temperatures, as of relevance to astronomical observations. The mass-selected ions are injected into a 22-pole radio frequency trap where internal degrees of freedom are deactivated by cryogenically cooled helium gas. In these first experiments the electronic transitions are detected by a predissociation process for N2O+ and para-dichlorobenzene cations (p-DCB+).In case of N2O+ the A˜Σ+2←X˜Π3/22 transition is rotationally resolved and the population fits to Trot∼25 K. The B˜B3u2←X˜B2g2 system of p-DCB+ shows narrow vibronic bands and an absence of hot bands indicating that the rotational and vibrational degrees of freedom have been relaxed. The vibrational structure has been assigned leading to the frequencies of several of the modes in the B˜B3u2 state. The dynamics of the photon absorption/dissociation are discussed.