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Photofragment spectroscopy of transient molecules

John Husband, University of Massachusetts Amherst


Photofragment spectroscopy is an extremely powerful technique capable of giving a wealth of information for molecular ions such as bond strengths, spectroscopic constants, molecular geometry and dissociation dynamics. Using a novel apparatus, we have obtained the photofragment spectra of MCH 2+ (M = Fe, Co, Ni), FeO+, and MS + (M = Fe, Ni). Each of these systems is important in the gas phase activation of methane. Brief introductions to gas-phase transition metal chemistry and to the experimental technique are given in Chapter 1. Chapter 2 contains the first detailed description of the home-built apparatus. ^ The information available from the photofragment spectra is found to be strongly dependent on the lifetime in the excited electronic state. For the MCH2+ systems studied, the lifetime is such that most spectroscopic information is lost. However, spectroscopic onsets are used to give rigorous upper limits to the M+-CH 2 bond strengths. Comparisons are made between the spectroscopic thresholds observed here and previously reported values of the M+-CH 2 bond strengths determined from guided ion-beam experiments. These results are given in Chapter 3. ^ In Chapter 4 the vibrationally resolved spectrum of FeO+ is shown. Analysis of this spectrum gives the first determination of vibrational frequency in both the ground electronic state and in the excited state. The 0 ← 0 band shows partially resolved rotational structure that is consistent with a 6Σ+6Σ + transition. Simulating the spectrum allows us to model the changes in rotational constants upon electronic excitation. ^ FeS+ and NiS+ are discussed in Chapter 5. Analysis of the FeS+ spectrum reveals it to arise from a 6Π ← 6Σ charge-transfer transition, and gives the vibrational frequency and spin-orbit constant in the 6Π state. The onset of the NiS+ spectrum appears to occur in the middle of a vibrational progression, thus giving a precise measurement of the Ni+-S bond strength. Analysis of the spectrum for vibrational and spin-orbit structure leads to a tentative assignment of a 4Δ ground state, and suggests the spectrum is composed of overlapping transitions to two near-degenerate excited electronic states, most likely a 4Π and a 4Φ pair. ^

Subject Area

Physical chemistry

Recommended Citation

Husband, John, "Photofragment spectroscopy of transient molecules" (2001). Doctoral Dissertations Available from Proquest. AAI3012140.