The detection of the electronic spectrum of FeCl2 in the gas phase

Abstract

Transition metal dihalides are prototypical molecules which provide an excellent test of ligand-field and higher level theories. Most of them have open-shell electronic structures and complicated electronic spectra. Although they have been studied for a long time,1,2 it is only in recent years that definitive studies have been carried out through the recording of gasphase spectra at rotational resolution.3,4 FeCl2 is one member of this group. In this communication, we report the first high resolution study of this species

Rotational and vibrational structure in the 288nm band system of the FeCl2 radical

“…The identification of FeCl 2 as the carrier of the spectrum is based on arguments presented in a previous publication [4]. To these we can add that the ground state vibrational interval of 352.39(13) cm À1 determined by dispersing the laser-induced fluorescence from several vibronic bands in this system [5] is entirely consistent with that expected by analogy with CoCl 2 [6], NiCl 2 [2] and CuCl 2 [31].…”

“…Those studied so far include FeCl 2 [4,5], CoCl 2 [6], NiCl 2 [7], and CuCl 2 [8]. Our preliminary observations of the 288 nm system of FeCl 2 were reported in a recent communication [4]. The ground state vibrational structure of this molecule, determined by dispersed fluorescence spectroscopy, is the subject of a companion paper [5].…”

“…By closing the iris to 2 mm diameter, the typical linewidth of strong rotational transitions was reduced from 800 to 350 MHz for recording of the most intense bands; in the weaker bands the much reduced signal-tonoise ratio far outweighed any benefits from reduced linewidth and it was necessary to open the iris. As has been explained elsewhere [4], when using the iris it was important to ensure that the nozzle was aligned so that the portion of the expanding jet from which the fluorescence was collected was as close to orthogonal to the laser beam as possible. If the iris were set so that a region of the jet with non-zero velocity parallel to the laser beam was being sampled, the signals would all be Doppler shifted.…”

“…The details of the experiment have been given in full elsewhere [2,26] and a discussion of the merits of various production methods for gas-phase FeCl 2 is given in a previous publication [4].…”

“…It reveals extensive vibrational structure, dominated by two long progressions in the excited state symmetric stretching vibration. In a previous publication [4], we speculated that these were progressions in 2 quanta of the bending mode. It will be shown that the previous assignment was incorrect; these transitions involve the symmetric stretching vibration that is greatly reduced from its ground state value of 352 to 200 cm À1 in the excited state.…”