Detection Of Photofragments Research Articles

Enhanced sensitivity in H photofragment detection by two-color reduced-Doppler ion imaging

Two-color reduced-Doppler (TCRD) and one-color velocity map imaging (VMI) were used for probing H atom photofragments resulting from the ~243.1 nm photodissociation of pyrrole. The velocity components of the H photofragments were probed by employing two counterpropagating beams at close and fixed wavelengths of 243.15 and 243.12 nm in TCRD and a single beam at ~243.1 nm, scanned across the Doppler profile in VMI. The TCRD imaging enabled probing of the entire velocity distribution in a single pulse, resulting in enhanced ionization efficiency, as well as improved sensitivity and signal-to-noise ratio. These advantages were utilized for studying the pyrrole photodissociation at ~243.1 and 225 nm, where the latter wavelength provided only a slight increase in the H yield over the self-signal from the probe beams. The TCRD imaging enabled obtaining high quality H(+) images, even for the low H photofragment yields formed in the 225 nm photolysis process, and allowed determining the velocity distributions and anisotropy parameters and getting insight into pyrrole photodissociation.

An improved installation for high-resolution laser spectroscopy on ion beams

For many years, a complex installation has been developed in our laboratory for high-resolution laser spectroscopy of fast ion and neutral beams. Various results have been obtained for different species (H2, O2+, N2+, N2O+, and CO2+). The different spectroscopic techniques applicable to an accelerated beam are presented or recalled in this work to show the performance of the apparatus. Some difficulties in understanding the b4 Σ–g O2+ predissociation process led us to modify the molecular photofragment detection system and make it more efficient, as shown by the results presented. PACS No.: 39.30.+w

The dipole moment of HOCl in vOH=4

Pulsed laser excitation and photofragment detection methods are used to observe the 17 0,17←16 1,16 pure rotational transition within the v OH=4 vibrational state of HO 35Cl. Microwave frequency and Stark effect measurements give ν 0=27484.33(10) MHz and μ b =1.562(9) D. The dependence of μ b , which is approximately parallel to the OH bond, on the level of OH stretch excitation appears linear and is consistent with that of H 2O over the same 0–14 000 cm −1 energy range.

Vibronic structure and photodissociation dynamics of the à state of jet-cooled ammonia

Vibrationally mediated photodissociation action spectroscopy provides vibronic spectra of the à state of jet-cooled ammonia by detecting the H-atoms produced by the photodissociation of vibrationally excited molecules. Initial vibrational excitation to selected rotation-inversion levels in the N–H stretching fundamental changes the Franck–Condon factors for the subsequent electronic transition markedly. Analysis of the vibronic structure in the à state reveals a progression in both the umbrella and the bending modes and provides fundamental frequencies for the symmetric and antisymmetric stretching motions. Additional state selectivity in infrared–ultraviolet optical double resonance excitation combined with photofragment detection allows rovibronic analysis of the rapidly predissociating levels in the à state of ammonia. The lifetime for NH3(Ã) excited to four quanta of bending motion is as short as 13±4 fs.

Identification of CH3OH2+ and H3O+-centered cluster isomers from fragment-dependent vibrational predissociation spectra of H+(CH3OH)4H2O

Cluster isomers of H+(CH3OH)4H2O have been identified by vibrational predissociation spectroscopy in combination with mass-selected detection of photofragments. Ab initio calculations indicate that the cluster ion can exist in either CH3OH2+(CH3OH)3H2O or H3O+(CH3OH)4 isomeric forms. They can dissociate via a methanol loss or water loss channel, depending on the structure of the isomers. While water loss is the dominant channel of the dissociation, the methanol loss channel is readily accessible by the H3O+-centered cluster isomer. We demonstrate in this study that mass-selected detection of photofragments produced by vibrational excitation is an effective way of identifying cluster isomers in the gas phase.

In-situ gas-phase luminescence and time-of-flight mass spectroscopic detection of photofragments during photochemical synthesis of copper particles from bis(tert-butylacetoacetato)copper.

During the 308 nm laser-driven photochemical synthesis of Cu particles from bis(tert-butylacetoacetato)copper, gas-phase photogenerated intermediates are identified by luminescence and time-of-flight mass spectroscopies. Pure Cu deposits are obtained as homogeneous, granular 200 nm particles. In the gas phase, luminescent photoproducts are observed and atomic Cu, Cu2, and dissociated ligand are identified spectroscopically. In addition, mass spectroscopy identifies Cu atoms, the dissociated ligand, a monoligated complex, and fragments of the ligands. The implications of the photofragmentation that produces copper atoms and dimers for the laser-assisted production of the Cu deposits are discussed.

Initial state resolved electronic spectroscopy of HNCO: Stimulated Raman preparation of initial states and laser induced fluorescence detection of photofragments

Stimulated Raman excitation (SRE) efficiently prepares excited vibrational levels in the ground electronic state of isocyanic acid, HNCO. Photofragment yield spectroscopy measures the electronic absorption spectrum out of initially selected states by monitoring laser induced fluorescence (LIF) of either NCO (X 2Π) or NH (a 1Δ) photofragments. Near threshold, the N–H bond fission is predissociative, and there is well-resolved rotational and vibrational structure in the NCO yield spectra that allows assignment of Ka rotational quantum numbers to previously unidentified vibrational and rotational levels in the ν1 N–H stretch and ν3 N–C–O symmetric stretch fundamentals in the ground electronic state of HNCO. The widths of NCO yield resonances depend on the initial vibrational state, illustrating one way in which initial vibrational state selection influences dissociation dynamics. Initial excitation of unperturbed ν1 (N–H stretch) states leads to diffuse NCO yield spectra compared to excitation of mixed vibrational levels. The higher energy dissociation channel that produces NH (a 1Δ) has coarser structure near its threshold, consistent with a more rapid dissociation, but the resonance widths still depend on the initially selected vibrational state.

Probing the dynamics of weakly bound complexes using high-resolution laser spectroscopy

A combination of high-resolution infrared spectroscopy and photofragment detection methods has been used to study the vibrational predissociation of a number of weakly bound complexes at the state-to-state level. The goal of this research is to provide detailed information on the dissociation of these highly non-statistical systems that can be used to make comparisons with the results of emerging theoretical methods and to gain new insights into the nature of bond rupture. In this paper we discuss results for several systems which illustrate some of the new methods we have developed. In the case of the HF dimer, the photofragment angular distributions provide sufficient information to assign the internal state distributions of the fragments. The experimental results are compared with recently reported theoretical calculations. Preliminary experimental results are also reported for the HF–DF and DF–HF complexes which, when combined with previously reported theoretical calculations and the HF dimer results, reveal deficiencies in the existing potential-energy surfaces that are not evident from the available spectroscopy.For systems where the fragments have smaller rotational constants or accessible excited vibrational states, the angular distributions no longer provide a unique assignment of the internal state quantum numbers. Without such, the detail with which the photofragmentation process can be studied is greatly reduced. For this reason we have developed two new methods that greatly aid in determining such information. The first involves the use of a second infrared laser to probe the fragments spectroscopically, while the second method involves orienting the parent molecular complex using a large dc electric field prior to dissociating it. In this way the two fragments can be detected separately, reducing the congestion in the angular distribution. These methods will be illustrated with several examples.

Angular correlation between photofragment velocity and angular momentum measured by resonance enhanced multiphoton ionization detection

REMPI detection of photofragments is discussed as a probe technique to measure the angular distribution of the velocity as well as the angular correlation between velocity and angular momentum existing in fragments formed during a polarized photolysis. The symmetry properties of the REMPI probe process induced by polarized light are expressed in terms of sensitivity to the moments of the rotational angular momentum distribution and the angular state of the photofragment is described in the formalism of bipolar moments. Original explicit expressions of the ion velocity profiles are given with probe geometry characteristics as parameters for two kinds of fragment detection (time-of-flight mass spectrometry and angular measurements of the ion yield) and the consequences of the v–J correlation on the observed profiles are discussed. Some experimental geometries are proposed in order to either determine the angular correlation or to avoid its effects.

Spectroscopy of the benzene cation: Resonance-enhanced multiphoton dissociation spectra of the B̃(E 2g)←X̃(E 1g) transition

We present the first well-resolved laser spectra of the lowest, dipole-forbidden electronic transition in the benzene radical cation (B̃(E 2g)←X̃(E 1g)). We employ our technique of resonance-enhanced multiphoton dissociation spectroscopy (REMPDS) which is unique in its capability of investigating bound, non-fluorescing ionic states with photofragment detection. The technique of multiphoton ionization permits the preparation of the ions either in the vibrationless ionic ground state or with one quantum of ν 16 excited. We thus obtained two ion spectra, representing vibronic levels of different symmetry. Most of the observed transitions are assignable to combination levels involving the inducing modes ν 16 and ν 17. Our results support recent theoretical work about Jahn-Teller and pseudo-Jahn-Teller effects in the B̃(E 2g) state.

Spectroscopy of molecular ions: Laser-induced fragmentation spectra of COS +, A 2Π ← X 2Π and B 2Σ ← X 2Π

A new method for ion spectroscopy is introduced which is particularly useful for polyatomic ions. The ions are mass selected, and spectroscopically analyzed by a pulsed dye laser employing photofragment detection subsequent to absorption. The sensitivity of this apparatus permits the measurement of photofragmentation spectra with a conventional, broadly tunable pulsed dye laser system in conjunction with a novel reflecting ion mass analyzer. Vibronic spectra of the transitions to both the A and B states of COS + are given, together with rotationally resolved spectra of the vibrationless, and of one vibrationally active, transition to the A state.

Theory of diatomic molecule photodissociation: Electronic angular momentum influence on fragment and fluorescence cross sections

We present a general theory of the photodissociation of diatomic molecules in the presence of nonadiabatic interactions between dissociative electronic states. Nonadiabatic couplings exert a profound influence on the photodissociation process when the molecule dissociates to atoms with nonvanishing electronic angular momentum, even if there are no ordinary curve crossings. Nonadiabatic interactions in this latter situation couple adiabatic molecular states which would otherwise be degenerate at infinite internuclear separation. Methods are developed for properly including nonadiabatic interactions in an exact or approximate calculation of photodissociation transition amplitudes for the production of the atomic fragments in particular fine structure levels. We derive differential cross sections for photofragment production and general expressions describing the detection of fragments by any secondary process, such as spontaneous emission, occurring during or after the breakup of the diatom. These expressions are then specialized to fragmentation of a diatom into atoms in fine structure states. Detection of photofragments, fluorescence from atoms produced in excited electronic states, or coincidence detection of both fragments and fluorescence, are considered in detail. An approximate treatment of nonadiabatic interactions is developed which is valid when the fragments recede with relative kinetic energy much greater than the magnitude of the nonadiabatic coupling matrix elements. In some cases, the effects of nonadiabatic interactions are still observable in the high kinetic energy limit. Finally, we discuss how the effect of nonadiabatic interactions may be observed experimentally, and how this can be used to amplify the information to be gained from a photodissociation experiment. In particular, it is shown how the polarization of the atomic fragments contains information fingerprinting the participating intermediate dissociative molecular electronic states.

Detection of photofragments by multiphoton ionization with direct resolution of angular and time-of-flight distributions

In the first application of resonant multiphoton ionization as a spatially and temporally resolved probe of photofragmentation dynamics, we have determined the angular and velocity distributions of CH3 and NO2 products scattered by infrared multiphoton dissociation of CH3NO2 in a skimmed, supersonic molecular beam. Our experiemental procedure rotates the focus of the vertically incident MPI probe beam in the plane of crossed infrared laser and molecular beams, 5 mm from the scattering center. Product detection occurs only within the focal volume of the probe laser yielding a solid angle resolution for the present arrangement of 0.01 steradians. From the data we offer a preliminary estimate of 3000 cm−1 as the average energy released in center-of-mass recoil for infrared laser-induced decomposition of nitromethane.

State selective doppler spectroscopy in a molecular beam. An application to multiple photon dissociation

The method of Doppler spectroscopy has been developed and applied under crossed beam conditions to the infra-red multiple-photon dissociation process, using laser fluorescence photofragment detection. Testing experiments have been carried out on the MPD of CH 3NH 2 by pulsed CO 2 laser light, yielding the recoil energy of NH 2 fragments state selectively.

Photofragment detection of nitrogen dioxide

Photofragment detection of nitrogen dioxide

Time-of-Flight Spectroscopy of CO2 Photodissociation in the Vacuum Ultraviolet. Electron Emission from Cesium Surface by Metastable Singlet Oxygen Atoms

Time-of-flight spectroscopy has been employed in conjunction with metastable photofragment detection by electron emission from metal surfaces to study the photodissociation of CO2 in the vacuum ultraviolet at wavelengths longer than 1050 Å. Irradiation was performed by unfiltered light. However, some wave-length selection was provided by the CO2 absorption itself and by choice of window material, LiF and CaF2. The metastable detector was coated with cesium. The photodissociation, CO2 + hυ→CO(X 1Σ)+O*, has been observed where O* can be O(1S) and/or O(1D) only. The result implies that the deactivation of metastable singlet oxygen atoms at a cesium surface produces electron emission. The analysis of the flight time distribution of O* shows that in the wavelength range from about 1160–1050 Å more than 50% of the dissociation leads to internally excited CO(X 1Σ) with considerable amounts of vibration-rotation energy.