Anion photoelectron spectroscopy of iodine–carbon dioxide clusters

Abstract

Anion photoelectron spectroscopy has proved to be a powerful method for the study of molecular clusters because it combines mass-selectivity and reasonable spectral resolution. Anion photoelectron spectra have been reported for elemental and molecular clusters of the type Ai, in which the additional electron is delocalized over the entire anion cluster,‘-6 and for mixed clusters of the type X(M),, in which a distinct Xchromophore interacts with a “solvating” species, M.‘,s The X(M) ,, photoelectron spectra obtained thus far have exhibited the same vibrational and electronic features seen in the bare Xspectrum, although these features are typically shifted and broadened in the cluster anion spectra due to the X-/M interaction. In this Communication, we present photoelectron spectra of the anion clusters I(CO,) ,,, n = l-l 3, that are qualitatively different from the Ispectrum in that they show pronounced progressions in vibrational modes of the solvating CO1 molecules. These arise from perturbations to the solvent molecules by the Icore. While Markovich et al. 8(a) have obtained photoelectron spectra for I(CO,),, n = l-7, they did not observe any vibrational structure due to the lower resolution in their experiment. Our results represent a new level of detail in the study of “solute-solvent” interactions in clusters. The experimental apparatus, described in detail elsewhere,’ employs a pulsed molecular beam (2% HI/C!Oz) propagating at a right angle to a 1 keV electron beam to generate vibrationally and rotationally relaxed anion clusters of the form I(COz) n. The anions are injected into a Wiley-McLaren type time-of-flight mass spectrometer; a typical mass spectrum is shown in Fig. 1. Anion clusters of the desired mass are photodetached with a properly timed light pulse from a Nd:YAG laser. The fourth (266 nm; 4.66 eV) and fifth (213 nm; 5.82 eV) harmonics of the Nd:YAG were used for these experiments. Energy analysis of the photoelectrons is performed by time-of-flight over a one meter field-free flight tube using microchannel plate detection. The instrumental resolution of the apparatus is 11 meV at 0.65 eV and varies as a function of ( eKE)3’2 (eKE = electron kinetic energy). Photoelectron spectra of I-, I(C02), and I( C02) 2 taken at hv=4.66 eV are presented in Fig. 2. Figure 3 shows the photoelectron spectra of I-(C02)n, n= 1-13, taken with hv=5.82 eV. The 5.82 eV spectra of I(CO,) and I(C02)2, compared with the 4.66 eV spectra, demonstrate the dependence of the instrumental resolution on the eKE. The Ispectrum consists of two peaks corresponding to the 2P3/2 ground state and the 2P1i2 excited state of the iodine atom; the spin-orbit splitting is 0.943 eV. The I( C02), spectra, in contrast, consist of two groups of peaks. The two bands in each spectrum, separated by approximately the I atom spin-orbit splitting, represent transitions to different electronic states of the neutral cluster which, to first order, can be labeled I(2P3,2) * (CO,), and I( 2P1,2) * ( C02),. The peaks in the I( 2P312) * ( COa), band, when they can be resolved at all, are noticeably broader than those in the I( *PI,,) * ( C02) n band at lower eKE. This is partly, but not totally (see below), due to the poorer resolution of the band at higher eKE. The intensities of peaks at very low eKE ( ~0.3 eV) are reduced due to the cutoff function of the electron detector.’ The cluster anion spectra show two important trends relative to the bare Ispectrum. First, the spectral peaks are located at progressively lower eKE’s as the number of CO2 molecules is increased. This “solvent shift,” seen in other X-(M), photoelectron spectra,“’ arises from the stronger attraction of CO2 to Ithan to an I atom. The cluster size dependence of the solvent shift is shown in Fig. 4. The more novel characteristic of these spectra is the vibrational progression in each of the bands. The average progression spacing” in the I(CO,), spectra, 665 *90 cm-’ (82 meV), is essentially identical to the CO, bending