Photodissociation dynamics of indole in a molecular beam

Photodissociation of indole at 193 and 248 nm under collision-free conditions has been studied in separate experiments using multimass ion imaging techniques. H atom elimination was found to be the only dissociation channel at both wavelengths. The photofragment translational energy distribution obtained at 193 nm contains a fast and a slow component. Fifty-four percent of indole following the 193 nm photoexcitation dissociate from electronically excited state, resulting in the fast component. The rest of 46% indole dissociate through the ground electronic state, giving rise to the slow component. A dissociation rate of 6 x 10(5) s(-1), corresponding to the dissociation from the ground electronic state, was determined. Similar two-component translational energy distribution was observed at 248 nm. However, more than 80% of indole dissociate from electronically excited state after the absorption of 248 nm photons. A comparison with the potential energy surfaces from the ab initio calculation has been made.     

Photodissociation and photoisomerization of alpha-fluorotoluene and 4-fluorotoluene in a molecular beam

The photodissociation of jet-cooled alpha-fluorotoluene and 4-fluorotoluene at 193 and 248 nm was studied using vacuum ultraviolet (vuv) photoionization/multimass ion imaging techniques as well as electron impact ionization/photofragment translational spectroscopy. Four dissociation channels were observed for alpha-fluorotoluene at both 193 and 248 nm, including two major channels C6H5CH2F-->C6H5CH2 (or C7H7)+F and C6H5CH2F-->C6H5CH (or C7H6)+HF and two minor channels C6H5CH2F-->C6H5CHF+H and C6H5CH2F-->C6H5+CH2F. The vuv wavelength dependence of the C7H7 fragment photoionization spectra indicates that at least part of the F atom elimination channel results from the isomerization of alpha-fluorotoluene to a seven-membered ring prior to dissociation. Dissociation channels of 4-fluorotoluene at 193 nm include two major channels C6H4FCH3-->C6H4FCH2+H and C6H4FCH3-->C6H4F+CH3 and two minor channels C6H4FCH3-->C6H5CH2 (or C7H7)+F and C6H4FCH3-->C6H5CH (or C7H6)+HF. The dissociation rates for alpha-fluorotoluene at 193 and 248 nm are 3.3 x 10(7) and 5.6 x 10(5) s(-1), respectively. The dissociation rate for 4-fluorotoluene at 193 nm is 1.0 x 10(6) s(-1). An ab initio calculation demonstrates that the barrier height for isomerization from alpha-fluorotoluene to a seven-membered ring isomer is much lower than that from 4-fluorotoluene to a seven-membered ring isomer. The experimental observed differences of dissociation rates and relative branching ratios between alpha-fluorotoluene and 4-fluorotoluene may be explained by the differences in the six-membered ring to seven-membered ring isomerization barrier heights, F atom elimination threshold, and HF elimination threshold between alpha-fluorotoluene and 4-fluorotoluene.     

Fluorescence spectra of fluorobenzene cations excited in a supersonic molecular beam

Fluorescence spectra of several fluorobenzene cations in the gas phase have been observed following electron impact on a supersonic beam of the neutral molecule. The very low rotational temperature of the beam is not disturbed by the ionisation process, so the different vibronic states of the cation ar produced rotationally cold; the fluorescence spectra are therefore very simple as every vibronic band has been condensed into a very few rotational com Vibrational frequencies obtained from the analyses agree excellently with values from other techniques. The effects of Jahn—Teller distortion are cle in the spectra of C₆F₆⁺ and 1,3,5-C₆F₃H₃⁺.     

Photodissociation of Cl2SO at 248 and 193 nm in a molecular beam

A pulse molecular beam of Cl₂SO was photodissociated at 248 and 193 nm. The time-of-flight distributions were observed for the photofragments, Cl, ClSO and SO. The primary processes are Cl + ClSO (I), 2Cl + SO (II) and Cl₂ + SO (III). At 193 nm the measured translational energy distributions imply a vibrationally excited ClSO fragment in process (I), and a simultaneous dissociation in process (II). The relative quantum yield is φ_I < φ_II. At 248 nm a radical process (I) is dominant compared to a molecular process (III).     

Flow modulation comprehensive two-dimensional gas chromatography-mass spectrometry with a supersonic molecular beam

A new approach of flow modulation comprehensive two-dimensional gas chromatography-mass spectrometry (GC x GC-MS) with supersonic molecular beam (SMB) and a quadrupole mass analyzer is presented. Flow modulation uniquely enables GC x GC-MS to be achieved even with the limited scan speed of quadrupole MS, and its 20 ml/min column flow rate is handled, splitless, by the SMB interface. Flow modulation GC x GC-SMB-MS shares all the major benefits of GC x GC and combines them with GC-MS including: (a) increased GC separation capability; (b) improved sensitivity via narrower GC peaks; (c) improved sensitivity through reduced matrix interference and chemical noise; (d) polarity and functional group sample information via the order of elution from the second polar column. In addition, GC x GC-SMB-MS is uniquely characterized by the features of GC-MS with SMB of enhanced and trustworthy molecular ion plus isotope abundance analysis (IAA) for improved sample identification and fast fly-through ion source response time. The combination of flow modulation GC x GC with GC-MS with SMB (supersonic GC-MS) was explored with complex matrices such as diesel fuel analysis and pesticide analysis in agricultural products.     

Benzene clusters in a supersonic beam

A supersonic beam is employed to produce benzene clusters (C₆H₆) _n up ton=40. Mass analysis is achieved after two-photon ionization in a reflectron mass spectrometer. Photon energy is chosen so that the internal energy of the cluster ions is less than 700 meV and a slow decay on the µs time scale is observed. By an energy analysis with the reflecting field it is found that the elimination of one neutral benzene monomer is the favoured dissociation process of the cluster ions. Information about the dissociation pathways of the cluster ions is essential if one is to obtain neutral cluster abundances from the ion mass spectrum. Furthermore an experimental method is presented to obtain pure intermediate state (S ₁←S₀) spectra of selected clusters without interferences from the other clusters present in the molecular beam. This method is based on the observation of the metastable decay of the corresponding cluster ion. When the metastable signal is recorded as a function of photon energy it reflects theS ₁←S ₀ intermediate state spectrum. Spectra are presented for the benzene dimer, trimer, tetramer and pentamer.     

Long radiative lifetimes of SO2 in a collision-free supersonic molecular beam

Radiative lifetimes and delayed laser-induced fluorescence spectra of cold SO₂ were measured in the range 3000–3150 Å in a differentially pumped collision-free supersonic beam. Lifetimes between 9 and 300 μs were obtained. We show the existence of high-density, low-intensity, and long-lifetime states in the vicinity of the strong spectral transitions.     

Isotope selective two-step photoionization study of K2 in a supersonic molecular beam

The photoionization efficiency curves for ^39,39K₂ and ^39,41K₂ dimers excited to the B ¹Π_u electronic state have been measured using sequential two-photon-ionization techniques. Accurate determinations of nine photoionization thresholds yield an adiabatic ionization energy of 4.06073 ± 0.00016 eV. Autoionizing Rydberg states are assigned and analyzed. Autoionization rates are measured for several Rydberg vibronic states. The isotopic dependence of the autoionization structure is partially analyzed and molecular constants for the K⁺₂ (²Σ⁺_g) state are derived.     

Photodissociation of an I2 supersonic beam in the nozzle expansion region: Influence on cluster spectra

Iodine clusters are obtained by free expansion of iodine vapour. The I₂ molecule is then photodissociated by a cw laser in the nozzle expansion region. Hot dissociated 1 atoms partially prevent nucleation, allowing accurate probing of the nucleation region. Moreover, I atoms can act as germs for the formation of odd clusters I_2n-1, leading to an increase of the ratio (number of I_2n-1⁺/number of I_2n⁺).     

Lifetimes of electronically excited NO2 in a collision-free differentially pumped supersonic molecular beam

The lifetimes of rotationally cooled NO₂ were measured in the spectral region 5710–5960 Å in a differentially pumped collision- free supersonic molecular beam. Lifetime values (rotationally independent) between 25 and 260 μs were obtained. Over narrow spectral regions, the product of the fluorescence amplitude and the lifetime squared is vibrationally independent, as predicted by theory.     

Polymorphism and phase control in titanyl phthalocyanine thin films grown by supersonic molecular beam deposition

Polymorphism in the growth of titanyl phthalocyanine films on dielectric substrates has been systematically studied by UV absorption and micro-Raman analyses, correlating structure and optical properties. We explored different growth regimes as a function of substrate temperature and growth rate using hyperthermal seeded supersonic beams. We identify and discuss specific signatures in micro-Raman spectra specifically correlated to the different phases and demonstrate the unprecedented ability of growing crystalline films and controlling the relative abundance of the different phases (amorphous, phase I, and phase II) by the beam parameters. We envisage the very promising perspective of controlling polymorphism at low temperatures via supersonic beam growth, paving the way for better performing devices.     

Photodynamics of the excited states of trimethylamine in a supersonic beam

The photophysics of trimethylamine (TMA) and rare gas-TMA van der Waals molecules has been studied under supersonic beam conditions. Dual exponential fluorescence decays observed for excitation of the second excited singlet state (S₂) are attributed to a novel S₂-S₁ relaxation induced by the vibrational predissociation of van der Waals molecules.     

Spectral characterization in a supersonic beam of neutral chlorophyll a evaporated from spinach leaves

The observation of the light absorption of neutral biomolecules has been made possible by a method implemented for their preparation in the gas phase, in supersonically cooled molecular beams, based upon the work of Focsa et al. [C. Mihesan, M. Ziskind, B. Chazallon, E. Therssen, P. Desgroux, S. Gurlui, and C. Focsa, Appl. Surf. Sci. 253, 1090 (2006)]. The biomolecules diluted in frozen water solutions are entrained in the gas plume of evaporated ice generated by an infrared optical parametric oscillators (OPO) laser tuned close to its maximum of absorption, at ~3 μm. The biomolecules are then picked up in the flux of a supersonic expansion of argon. The method was tested with indole dissolved in water. The excitation spectrum of indole was found cold and large clusters of indole with water were observed up to n = 75. Frozen spinach leaves were examined with the same method to observe the chlorophyll pigments. The Q(y) band of chlorophyll a has been observed in a pump probe experiment. The Q(y) bands of chlorophyll a is centred at 647 nm, shifted by 18 nm from its position in toluene solutions. The ionization threshold could also be determined as 6.1 ± 0.05 eV.     

Photofragment spectroscopy of HN3 cooled in a supersonic beam

The 248 nm photolysis of HN₃, cooled to ≈ 3 K by expansion in a supersonic beam, producing NH is studied. The resulting nascent rotational and vibrational distributions of the NH(a^≈1 Δ) are reported. Comparisons at 248 and 266 nm using room-temperature HN₃ are made. A dependence of the final rotational state distributions (NH) on the initial state distributions (HN₃) is found.     

A supersonic beam of cold lithium hydride molecules

We have developed a source of cold LiH molecules for Stark deceleration and trapping experiments. Lithium metal is ablated from a solid target into a supersonically expanding carrier gas. The translational, rotational, and vibrational temperatures are 0.9+/-0.1, 5.9+/-0.5, and 468+/-17 K, respectively. Although they have not reached thermal equilibrium with the carrier gas, we estimate that 90% of the LiH molecules are in the ground state, X (1)Sigma(+)(v=0,J=0). With a single 7 ns ablation pulse, the number of molecules in the ground state is 4.5+/-1.8 x 10(7) molecules/sr. A second, delayed, ablation pulse produces another LiH beam in a different part of the same gas pulse, thereby almost doubling the signal. A long pulse, lasting 150 micros, can make the beam up to 15 times more intense.     

A supersonic molecular beam study of the reaction of tetrakis(dimethylamido)titanium with self-assembled alkyltrichlorosilane monolayers

The reaction of a transition metal coordination complex, Ti[N(CH(3))(2)](4), with self-assembled monolayers (SAMs) possessing-OH, -NH(2), and -CH(3) terminations has been examined using supersonic molecular beam techniques. The emphasis here is on how the reaction probability varies with incident kinetic energy (E(i)=0.4-2.07 eV) and angle of incidence (theta(i)=0 degrees -60 degrees ). The most reactive surface is the substrate underlying the SAMs-SiO(2) with a high density of -OH(a) (>5 x 10(14) cm(-2)), "chemical oxide." On chemical oxide, the dynamics of adsorption are well described by trapping, precursor-mediated adsorption, and the initial probability of adsorption depends only weakly on E(i) and theta(i). The dependence of the reaction probability on substrate temperature is well described by a model involving an intrinsic precursor state, where the barrier for dissociation is approximately 0.2-0.5 eV below the vacuum level. Reaction with the SAMs is more complicated. On the SAM with the unreactive, -CH(3), termination, reactivity decreases continuously with increasing E(i) while increasing with increasing theta(i). The data are best interpreted by a model where the Ti[N(CH(3))(2)](4) must first be trapped on the surface, followed by diffusion through the SAM and reaction at the SAMSiO(2) interface with residual -OH(a). This process is not activated by E(i) and most likely occurs in defective areas of the SAM. On the SAMs with reactive end groups, the situation is quite different. On both the-OH and -NH(2) SAMs, the reaction with the Ti[N(CH(3))(2)](4) as a function of E(i) passes through a minimum near E(i) approximately 1.0 eV. Two explanations for this intriguing finding are made-one involves the participation of a direct dissociation channel at sufficiently high E(i). A second explanation involves a new mechanism for trapping, which could be termed penetration facilitated trapping, where the Ti[N(CH(3))(2)](4) penetrates the near surface layers, a process that is activated as the molecules in the SAM must be displaced from their equilibrium positions.     

Photodissociation of high molecular weight peptides and proteins in a two-stage linear time-of-flight mass spectrometer

A two-stage linear time-of-flight mass spectrometer is used to investigate the requirements for performance of laser photodissociation of peptide and protein ions. Results are presented that demonstrate that desorption and dissociation laser pulses can be synchronized to irradiate ions that travel at high velocities down the drift tube of a time-of-flight mass spectrometer. For example, 193-nm photodissociation of bovine insulin and doubly charged lysozyme is demonstrated, and laser power studies suggest that dissociation is initiated by the absorption of a single 193-nm photon. These results are encouraging because they suggest that laser photodissociation of high molecular weight proteins can lead to fragmentation on time scales compatible with time-of-flight mass spectrometry.     

Reaction rate for isomerization in a molecular beam

It is shown that in an ideal gas (molecular beam), at temperature T, consisting of molecules that can isomerize and whose molecular dynamics can be non-ergodic, the observed decay rate of the concentration can be greater than that. k_TST, predicted by transition state theory. The “observed” rate constant is then given by (Q/Q_≠⁽¹⁾)k_TST where Q_≠⁽¹⁾ is the contribution to the canonical partition function Q arising from only activated and irregular states. It is also shown that in the presence of strong collisions the long-time decay rate of the concentration provides no information about the intramolecular rate process, but at low collision rates it is sensitive to the non-ergodic dynamics.     

Photodissociation of molecular beams of halogenated hydrocarbons at 193 nm

Molecular beams of halogenated hydrocarbons containing chlorine and bromine atoms were photodissociated using an excimer laser at 193 nm. Molecules photodissociated were HCCBr, HCCCH₂Br, HCCCH₂Cl, CH₃Cl, C₂H₅Cl and i-C₃H₇Cl. The time-of-flight distributions of the photofragments were measured in order to study the primary processes and the dissociation dynamics. Generalizations consistent with the data are that atomic products (RX → R + X) result from direct dissociation of the CX repulsive singlet state, molecular elimination (RX → R′ + HX) is a result of a crossover to the ground state and triplet states are involved in the photodissociation of alkyne compounds.