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The problem for molecular identification knows many solutions, which include mass spectrometers whose mass sensitivity depends on the performance of the detector involved. The purpose of this article is to show by means of molecular dynamics simulations how a laser-cooled ion cloud, confined in a linear radio-frequency trap, can reach the ultimate sensitivity providing the detection of individual charged heavy molecular ions. In our simulations, we model the laser-cooled Ca+ ions as two-level atoms, confined thanks to a set of constant and time oscillating electrical fields. A singly charged molecular ion with a mass of 106 amu is propelled through the ion cloud. The induced change in the fluorescence rate of the latter is used as the detection signal. We show that this signal is due to a significant temperature variation triggered by the Coulomb repulsion and amplified by the radio-frequency heating induced by the trap itself. We identify the optimum initial energy for the molecular ion to be detected, and furthermore, we characterize the performance of the detector for a large range of confinement voltages. [ABSTRACT FROM AUTHOR]

A study of a deuterium atom transfer in the excited state of perdeuterated phenol - (ND3) n clusters as well as its comparison to the hydrogen atom transfer process in phenol- (NH3) n=1,4 cluster excited state is presented here. These experimental observations give an alternative to the excited state proton-transfer dynamics paradigm and shows that the reaction is not greatly affected by addition of small amount of water.

The phenol-(NH3)(sub n=123)(S1) lifetimes are measured near their band origins by a picosecond pump/probe scheme. The very short lifetimes observed revealed that excited clusters undergo a reactive process namely a issociative hydrogen transfer reaction.

Research was conducted to examine the solvent effect on the appearance of a red-shifted twisted intramolecular charge-transfer (TICT) emission in molecular clusters. Dimethylaminobenzomethyl ester-acetronitrile clusters were investigated by monitoring time mass spectra and dispersed fluorescence spectra and lifetime measurements using a supersonic expansion. Results indicate that the rate of transfer from the locally excited state to the TICT state increases as the cluster size increases.

Lifetimes of the first electronic excited state (S1) of fluorine and methyl (o-, m-, and p-) substituted phenols and their complexes with one ammonia molecule have been measured for the 00 transition and for the intermolecular stretching σ1 levels in complexes using picosecond pump-probe spectroscopy. Excitation energies to the S1 (ππ*) and S2 (πσ*) states are obtained by quantum chemical calculations at the MP2 and CC2 level using the aug-cc-pVDZ basis set for the ground-state and the S1 optimized geometries. The observed lifetimes and the energy gaps between the ππ* and πσ* states show a good correlation, the lifetime being shorter for a smaller energy gap. This propensity suggests that the major dynamics in the excited state concerns an excited state hydrogen detachment or transfer (ESHD/T) promoted directly by a S1/S2 conical intersection, rather than via internal conversion to the ground-state. A specific shortening of lifetime is found in the o-fluorophenol-ammonia complex and explained in terms of the vibronic coupling between the ππ* and πσ* states occurring through the out-of-plane distortion of the C-F bond. [ABSTRACT FROM AUTHOR]

Phenol-ammonia clusters with more than five ammonia molecules are proton transferred species in the ground state. In the present work, the excited states of these zwitterionic clusters have been studied experimentally with two-color pump probe methods on the nanosecond time scale and by ab initio electronic-structure calculations. The experiments reveal the existence of a long-lived excited electronic state with a lifetime in the 50–100 ns range, much longer than the excited state lifetime of bare phenol and small clusters of phenol with ammonia. The ab initio calculations indicate that this long-lived excited state corresponds to a biradicalic system, consisting of a phenoxy radical that is hydrogen bonded to a hydrogenated ammonia cluster. The biradical is formed from the locally excited state of the phenolate anion via an electron transfer process, which neutralizes the charge separation of the ground state zwitterion. [ABSTRACT FROM AUTHOR]

Photoexcitation of protonated aromatic amino acids leads to Cα[Single_Bond]Cβ bond breakage among other channels. There are two pathways for the Cα[Single_Bond]Cβ bond breakage, one is a slow process (microseconds) that occurs after hydrogen loss from the electronically excited ion, whereas the other is a fast process (nanoseconds). In this paper, a comparative study of the fragmentation of four molecules shows that the presence of the carboxylic acid group is necessary for this fast fragmentation channel to occur. We suggest a mechanism based on light-induced electron transfer from the aromatic ring to the carboxylic acid, followed by a fast internal proton transfer from the ammonium group to the negatively charged carboxylic acid group. The ion formed is a biradical since the aromatic ring is ionized and the carbon of the COOH group has an unpaired electron. Breakage of the weak Cα[Single_Bond]Cβ bond gives two even-electron fragments and is expected to quickly occur. The present experimental results together with the ab initio calculations support the interpretation previously proposed. [ABSTRACT FROM AUTHOR]

The photofragmentation of protonated tryptophan has been investigated in a unique experimental setup, in which ion and neutral issued from the photofragmentation are detected in coincidence, in time and in position. From these data are extracted the kinetic energy, the number of neutral fragments associated with an ion, their masses, and the order of the fragmentation steps. Moreover, the fragmentation time scale ranging from tens of nanoseconds to milliseconds is obtained. From all these data, a comprehensive fragmentation mechanism is proposed. [ABSTRACT FROM AUTHOR]

Ar2+ ions produced in a cooled supersonic expansion by electron-impact ionization are accelerated at 2.5 keV and kept during few milliseconds inside a linear electrostatic trap. The lifetime of the metastable Ar2+ ion is determined from the measurement of the rate of the argon atoms escaping the trap. The lifetime and the relative metastable populations are measured as a function of the pressure and temperature in the supersonic expansion, i.e., of the mean cluster size. Possible mechanisms responsible for the metastable formation are discussed. [ABSTRACT FROM AUTHOR]

The electronic excited state dynamics of protonated tryptamine ions generated by an electrospray source have been studied by means of photoinduced dissociation technique on the femtosecond time scale. The result is that the initially excited state decays very quickly within 250 fs. The photoinduced dissociation channels observed can be sorted in two groups of fragments coming from two competing primary processes on the singlet electronic surface. The first one corresponds to a hydrogen-atom loss channel that creates a tryptamine radical cation. The radical cation subsequently fragments to smaller ions. The second process is internal conversion due to the H-atom recombination on the electronic ground state. Time-dependent density functional theory calculations show that an excited πσ* state dissociative along the protonated amino N–H stretch crosses both the locally excited ππ* state and the electronic ground state S0 and thus triggers the photofragmentation reactions. The two processes have equivalent quantum yields, ≈50% of the fragments coming from the H-atom loss reaction. The two primary reaction paths can clearly be distinguished by their femtosecond pump/probe dynamics recorded on the different fragmentation channels. [ABSTRACT FROM AUTHOR]

The excited state hydrogen atom transfer reaction (ESHT) has been studied in pyrrole–ammonia clusters [PyH–(NH3)n+hν→Py•+•NH4(NH3)n-1]. The reaction is clearly evidenced through two-color R2P1 experiments using delayed ionization and presents a threshold around 235 nm (5.3 eV). The cluster dynamics has also been explored by picosecond time scale experiments. The clusters decay in the 10–30 ps range with lifetimes increasing with the cluster size. The appearance times for the reaction products are similar to the decay times of the parent clusters. Evaporation processes are also observed in competition with the reaction, and the cluster lifetime after evaporation is estimated to be around 10 ns. The kinetic energy of the reaction products is fairly large and the energy distribution seems quasi mono kinetic. These experimental results rule out the hypothesis that the reaction proceeds through a direct N–H bond rupture but rather imply the existence of a fairly long-lived intermediate state. Calculations performed at the CASSCF/CASMP2 level confirm the experimental observations, and provide some hints regarding the reaction mechanism. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We propose a very simple and efficient way to stabilize ions issued from a collision complex through a femtosecond coherently controlled pump/probe process. Starting from a van der Waals complex, one can initiate a collision at a well-defined time and with a restricted impact parameter. Formation of stable ionic complex can be achieved by ionizing the collision complex at the "right time." We present in this paper its application to the NaI-(CH[SUB3]CN)[SUB1-2] system. Na[SUP+]-CH[SUB3]CN ion formation is coherently controlled by ionization of colliding Na atom on CH[SUB3]CN molecules issued from the dissociation of NaI within NaI-(CH[SUB3]CN)[SUB1-2]. Classical mechanic calculations using simple ionization/dissociation conditions can reproduce the experimental data and give an insight into the control of such a reaction. [ABSTRACT FROM AUTHOR]

The excited-state dynamics of protonated tryptophan ions is investigated by photoinduced fragmentation in the gas phase. The result shows that the bond-cleaving reactions can be controlled by a variation of the pump or probe delay.

The high repulsive states of HgAr and HgNe van der Waals complexes, correlating with Hg 6s6d atomic states have been investigated by double resonance spectroscopy, through the first excited state A 30+ and B 31 of the complexes. The repulsive potentials have been fitted through numerical Franck–Condon simulations. They have been characterized by perturbative calculation as quasi-pure 6dΣ potentials in Hund’s case a. The strong Hg–rare gas electrostatic interaction potential overruns the spin–orbit interaction at distances shorter than 7 Å. These observed repulsive states are mostly of Ω=1 character correlating with 3D3 at infinite distances. The contribution from the potential of Ω=0- symmetry correlating with 1D2 is of minor importance. Therefore, the absorption in the repulsive states of the complex arises mostly from proximity induced absorption in an optically forbidden transition 3P1→3D3. A perturbative model accounts well for the bound free absorption intensities experimentally observed. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Mass resolved resonance enhanced excitation spectra of HgArn have been measured in the vicinity of the Hg(6s6p) 3P1→(6s2) 1S0 transition. The spectra are compared with calculations based on additive potentials for both the ground and the excited state. These potentials give reasonable agreement with the experiments. Deviation from this simple model occurs in the excited electronic state mainly for structures where the argon atoms are in the closest approach from the Hg. [ABSTRACT FROM AUTHOR]

Fluorescence excitation spectra of silver–argon van der Waals complexes are presented. An alternative experimental technique to laser evaporation has been used to produce the cold complexes. The van der Waals states observed correlate with the (4d10 5s1) 2S Ag ground state and (4d10 5p1)2P0 Ag first excited state. The excited state of the Ag–Ar complex is found to be 1235 cm-1 deep, which is unusually large for such metal–rare gas complexes. A perturbation due to a state correlating with the (4d9 5s2)2D metastable Ag state is also observed and analyzed. [ABSTRACT FROM AUTHOR]

We report here the fluorescence excitations spectrum of the Si–Ar van der Waals complex in the region of the (3p4s)3P–(3p2)3P atomic transition. Long progressions are observed, which have been assigned to a Π–Σ transition. The potential curves derived from the analysis of these progressions are discussed in terms of effects of spin–orbit coupling on van der Waals interactions. [ABSTRACT FROM AUTHOR]

The present work has addressed the question of the dissociation time of four alkyl nitrites upon photoexcitation in the S1 state (methyl nitrite, n- , and t-butyl nitrites and i-amyl nitrite). The time resolved Laser Induced Fluorescence technique has been used in the femtosecond regime under bulk conditions. The photodissociation has been initiated at 351 nm by 150 fs pump pulses, and has been probed using a two-photon process at 467 nm by 200 fs pulses. The LIF signal has qualitatively the same shape for the four nitrites: it passes through a transient peak before reaching a plateau. The two-photon process that induces the detected fluorescence is nonresonant for detection of the dissociation product NO through the A←X transition. Conversely, the two-photon process is resonant or quasiresonant for detection of the excited nitrite molecule in the S1 level before it dissociates. This leads to an enhanced detection efficiency of the non-dissociated excited molecule versus that of the NO fragment. A simple kinetic model has been developed to account for this detection scheme. It shows that the transient peak is observable only if the lifetime of the excited molecule is comparable or larger than the temporal width of the lasers. In that case, the model allows the determination of an effective lifetime of the excited molecule from the measured LIF signal. Lifetimes have been found in the range of 125 fs for the four nitrites investigated. This has allowed us to answer a controversy on the dissociation mechanism of methyl nitrite and to confirm that an indirect pathway exists in the photodissociation of this molecule. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]