The fluorescence and photodissociation of rhodamine 575 cations confined to a quadrupole ion trap are observed during laser irradiation at 488 nm. The kinetics of photodissociation is measured by time-dependent mass spectra and time-dependent fluorescence. The rhodamine ion signal and fluorescence decay are studied as functions of buffer gas pressure, laser fluence, and irradiation time. The decay rates of the ions in the mass spectra agree with decay rates of the fluorescence. Some of the fragment ions also fluoresce and further dissociate. The photodissociation rate is found to depend on the incident laser fluence and buffer gas pressure. A s an alternative to collision-induced dissociation, surface-induced dissociation, infrared multiphoton dissociation, or direct UV/visible dissociation of biomolecular ions, there has been interest in labeling biomolecules with a chromophore tag to enhance photoactivation efficiency and selectivity. For example, Brodbelt and colleagues studied photodissociation mass spectra to sequence peptides labeled with both infrared absorbers [1] and UV chromophores [2]. Fluorophore labeling is also used in fluorescence resonant energy transfer (FRET) experiments, in which the acceptor/donor interactions of two attached fluorophores depend on the distances between them and therefore can provide conformational information [3][4][5][6]. In gas-phase work, Parks and coworkers [7][8][9][10] and Zenobi and coworkers [11,12] investigated the fluorescence of trapped molecular ions commonly used as fluorescent molecular probes of biomolecules. Both groups attached fluorescent probes to peptide cations for the purposes of investigating gas-phase FRET [8,11].Chromophore and fluorophore labeling experiments both rely on energy transfer from the absorbing moiety to another part of the biomolecule, either via intramolecular vibrational energy transfer or via radiative or electronic interactions. To understand the initial absorption process and the efficiency of photoactivation, it is of interest to investigate the photophysics and chemical dynamics of the absorption, fluorescence, and dissociation processes in the fluorescent dye molecules themselves in the gas phase. This work investigates the competition between fluorescence and photodissociation of rhodamine dye cations in a quadrupole ion trap. The ion trap is an ideal device for measuring chemical reactions that have slow reaction rates or processes that have long-lived intermediate states because of its ability to store a population of thermalized ions in the same location for extended periods of time. The photodepletion rate of the rhodamine cations is determined here by two independent methods: by directly observing the fluorescence photons and by recording the ion mass spectra. The resulting decays obtained by both methods are compared as a function of pressure, laser fluence, and irradiation time. The mechanism of rhodamine cation photodissociation at 488 nm is discussed.Several other groups have directly measured fluorescence from ions in...

The photodissociation of rhodamine 575 cations held in a quadrupole ion trap is studied using 514 nm light as a function of buffer gas pressure, irradiation time, and laser fluence. The laser-induced photodissociation decays of rhodamine ions have lifetimes on the order of seconds for the range of pressures and powers investigated and exhibit strong nonlinear pressure dependence. Dissociation mechanisms are considered that involve the sequential absorption of multiple photons and several collisional deactivation steps.

Pulsed extraction techniques are investigated for a quadrupole ion trap (QIT) interfaced to a linear time-of-flight (TOF) mass analyzer. A nonfocusing short-pulse mode of operation is developed and characterized. The short-pulse mode creates a near-monoenergetic ion packet, which is useful for reaction kinetics experiments and for making diagnostic measurements of the ion cloud size in the trap. Monopolar and bipolar pulsing modes, with the voltage pulses applied to one or both QIT endcaps to extract the ions into the TOF region, are compared. Ion TOF peak distributions are characterized experimentally and by ion trajectory simulations. Also, first-order spatial (Wiley-McLaren) focusing of ions is characterized for the conventional long-pulse extraction mode. The nonparallel fields in the QIT, which serves as the first acceleration region in the linear-TOF mass spectrometer, are shown to degrade spatial focusing and mass resolution.

The full state-resolved distribution of scattered CO2 (00(0)0) molecules from collisions with highly vibrationally excited pyrazine (E = 32,700 cm(-1)) is reported and compared to previous studies on pyrazine (E = 37,900 cm(-1)) to investigate how internal energy content impacts the dynamics for collisional quenching of high energy molecules [J. Phys. Chem. A 2010, 113, 1569]. Nascent rotational and translational energy profiles for scattered CO2 (00(0)0) molecules with J = 2-72 were measured using high-resolution transient infrared absorption and combined with earlier results for the J = 56-78 states [J. Chem. Phys. 1999, 111, 7373]. The product translational energy for individual J-states increases by 50% for a 16% increase in donor vibrational energy. The nascent rotational distribution for scattered CO2 is biexponential, comprising 77% nearly elastic collisions and 23% inelastic collisions. The spread of the rotational distribution is sensitive to donor energy, but the branching ratio for elastic and inelastic collisions is the same for both donor energies. The measured collision rates are close to the Lennard-Jones values and are only weakly dependent on changes in donor energy. The nascent energy gain distribution function P(ΔE) depends strongly on the energy, and this energy dependence is stronger than the linear dependence seen in multicollision energy transfer studies for pyrazine(E) + CO2 collisions.