Ultraviolet photodissociation dynamics of PCl3 at 235 nm: three-dimensional ion imaging and theoretical analysis.

The photodissociation dynamics of PCl3 at 235 nm has been studied by monitoring ground state Cl(2P3/2) and spin-orbitally excited Cl(2P1/2) atoms by resonance enhanced multiphoton ionization (REMPI). Also, the PCln+ (n = 0, 1, 2) photoions were observed non-resonantly. The speed distributions and speed-dependent anisotropy parameters ß for all these particles have been determined by three-dimensional photofragment ion imaging. The ß parameters are close to zero for all these particles. The relative yield of excited Cl(2P1/2) atoms for photodissociation of PCl3 is 0.49 ± 0.03. The speed distributions of Cl(2P3/2) and Cl(2P1/2) atoms are bimodal, mainly resulting from the sequential photodissociation PCl3 → PCl2 → PCl. Near-resonant two-photon ionisation followed by near-resonance one-photon photodissociation is proposed for the production of PCln+ photoions. Using Condon's reflection principle, we constructed the vibrational mode dependent absorption spectrum by accounting for the vibrational motion for all six normal coordinates. As a result, the absorption of 235 nm radiation by PCl3 occurs due to zero vibrational motion rather far from the equilibrium geometry. In particular, movement along the Q2 normal coordinate results in an efficient photoinduced transition into the excited state à of D3h symmetry due to a parallel transition.

Double-arm three-dimensional ion imaging apparatus for the study of ion pair channels in resonance enhanced multiphoton ionization.

We present a novel experimental configuration for the full quantitative characterization of the multichannel resonance enhanced multiphoton ionization (REMPI) of small molecules in cases when the ion-pair dissociation channel is important. For this purpose, a double-arm time-of-flight mass spectrometer with three-dimensional (3D) ion imaging detectors at both arms is constructed. The REMPI of HCl molecules is used to examine the constructed setup. The apparatus allows us to perform simultaneous measurements of the 3D velocity vector distributions of positive (H(+), HCl(+), and Cl(+)) and negative (Cl(-)) photoions. The characterization consists of the determination of "two-photon absorption cross sections" for the process HCl(X)+2hν → HCl*, one-photon absorption cross sections for subsequent processes HCl* + hν → HCl*, and the probability of the subsequent non-adiabatic transition HCl* → HCl(B) → H(+) + Cl(-), which leads to ionic pairs. All these data should be obtained from the analysis of the dependencies of the number of ions on the laser energy. The full characterization of the laser beam and the knowledge of the ion detection probability are necessary parts of the analysis. Detailed knowledge of losses of produced ions in the mass spectrometer before detection requires understanding and characterization of such processes like electron emission from metallic grids under ion bombardment or charge transfer between positive ions and the metal surface of the grids, like Cl(+) + (grid) → Cl(-). These important phenomena from surface science are rarely discussed in the imaging literature, and here, we try to compensate for this shortcoming.

Simultaneous imaging of both product ions: exploring gateway states for HCl as a benchmark molecule.

Simultaneous imaging of both positive and negative product ions is used to exclusively study photoion pair formation free from interference of competing fragmentation channels. Resonance enhanced multi-photon excitation allows us to interrogate potential energy surfaces for vastly differing molecular geometries. 3D imaging provides complete fragment information. We applied the technique to HCl as a benchmark and identified the gateway state leading to photoion pairs. The approach can easily be applied to any molecule exhibiting a potential with an attractive part at large internuclear distances.

Proton formation dynamics in the REMPI[2+n] process via the F 1Delta2 and f 3Delta2 Rydberg states of HCl investigated by three-dimensional velocity mapping.

HCl in the bulk gas phase at a pressure of 10(-5) mbar has been excited via selected Q-lines of the two-photon transition band systems F (1)Delta(2)<--X (1)Sigma(+)(1,0) [Q(8)], V (1)Sigma(+)<--X (1)Sigma(+)(14,0) [Q(8), Q(7)] and f (3)Delta(2)<--X (1)Sigma(+)(0,0) [Q(2-6)]. Concerning the V<--X excitation, subsequent photon absorption is known to yield HCl(+), H(n=2)+Cl, H(+)+Cl(-) and H+Cl(4s,4p,3d). Vibrationally excited HCl(+) (v(+) > or = 5) can be photodissociated to H(+)+Cl, and excited atoms can be easily photoionized by absorption of a fourth photon, respectively. Using three-dimensional velocity map imaging, the spatial proton velocity distributions resulting from these processes for these particular transitions were studied for the first time. Kvaran et al. [J. Chem. Phys. 131, 044324 (2009); J. Chem. Phys. 129, 164313 (2008)] recently reported a substantial increase in the formation of chlorine and hydrogen ions in single rovibrational transitions of the F (1)Delta(2) and f (3)Delta(2) band systems using mass resolved resonance enhanced multiphoton ionization spectroscopy and explained this by the vicinity of single rovibrational levels of the V (1)Sigma(+) state for which photorupture is the main feature. Thus, the known dissociation dynamics of the V (1)Sigma(+) state should also leave their fingerprint in the spatial proton velocity distribution emerging from the photodissociation of those states. Accordingly, we found a strong increase in the H(+) ion signal for the Q(5) line of the f (3)Delta(2)<--X (1)Sigma(+)(0,0) transition, the extra signal resulting from dissociation into H(n=2)+Cl((2)P(1/2)) and the ion pair. No increase for the HCl(+)(v(+) > or = 5) photodissociation channel or dissociation into H(n=2)+Cl((2)P(3/2)) has been observed. Furthermore, H(+) distributions from the Q transitions of the f (3)Delta(2)<--X (1)Sigma(+)(0,0) band system were found to show the two features previously ascribed to the "gateway" state [(4)Pi...4s](3)Pi(0), i.e., autoionization into HCl(+)(5 < or = v(+) < or = 8) and nonadiabatic dissociation into H(n=2)+Cl((2)P(3/2)). The F (1)Delta(2)<--X (1)Sigma(+)(1,0) band system only showed significant proton formation for the Q(8) line. The speed distribution is the same as for the Q(8,7) lines of the V (1)Sigma(+)<--X (1)Sigma(+)(14,0) transition while the excitation history is conserved in the angular distribution confirming the resonance interpretation.

Measurement of the differential cross section of the photoinitiated reactive collision of O((1)D)+D(2) using only one molecular beam: A study by three dimensional velocity mapping.

In order to measure the state selective double differential cross section of a reactive collision, the preparation of the reactants with defined initial velocities and quantum states in number densities high enough to achieve an acceptable count rate is most important. At the same time, secondary collisions have to be prevented in order to ensure that the nascent products are not thermalized. Usually, the best way to control the initial conditions is to use crossed molecular beams, but the number density decreases quadratically with the distance from the nozzle orifice which can be a problem, especially if a molecular product with a large number of populated states is to be analyzed state specifically by REMPI spectroscopy. In this contribution we would like to present a method for measuring the quantum state selective differential cross section of a photoinitiated reaction that combines the advantages of the PHOTOLOC technique (high reactant densities) and the parallel beams technique used by the groups of Kitsopoulos, Orr-Ewing, and Suits (defined relative velocity of the reactants). Moreover, an algorithm based on a Bayesian backward reconstruction developed by W. H. Richardson [J. Opt. Soc. Am. 62, 55 (1972)] has been derived. Both, one reactant and the precursor of the other reactant, are present in the same molecular beam and the center of mass velocity is selected by shifting the dissociation and the detection laser in time and space. Like in comparable methods, this produces a bias in the measured velocity distribution due to the fact that the reaction takes place in the whole volume surrounding the laser beams. This has been also reported by Toomes et al. in the case of the parallel beams technique and presents a general problem of probing reaction products by REMPI spectroscopy. To account for this, we develop a general approach that can be easily adapted to other conditions. The bias is removed in addition to deconvolution from the spread in reactant velocities. Using the benchmark system O((1)D)+D(2) with N(2)O as the precursor, we demonstrate that the technique is also applicable in a very general sense (i.e., also with a large spread in reactant velocities, products much faster than reactants) and therefore can be used also if such unfortunate conditions cannot be avoided. Since the resulting distribution of velocities in the laboratory frame is not cylindrically symmetric, three dimensional velocity mapping is the method of choice for the detection of the ionized products. For the reconstruction, the distance between the two laser beams is an important parameter. We have measured this distance using the photodissociation of HBr at 193 nm, detecting the H atoms near 243 nm. The collision energy resulting from the 193 nm photodissociation of N(2)O is 5.2+/-1.9 kcal/mol. Our results show a preference for backward scattered D atoms with the OH partner fragment in the high vibrational states (v=4-6), in accord with previously published results claiming the growing importance of a linear abstraction mechanism for collision energies higher than 2.4 kcal/mol.

Three-dimensional velocity map imaging: setup and resolution improvement compared to three-dimensional ion imaging.

For many years the three-dimensional (3D) ion imaging technique has not benefited from the introduction of ion optics into the field of imaging in molecular dynamics. Thus, a lower resolution of kinetic energy as in comparable techniques making use of inhomogeneous electric fields was inevitable. This was basically due to the fact that a homogeneous electric field was needed in order to obtain the velocity component in the direction of the time of flight spectrometer axis. In our approach we superimpose an Einzel lens field with the homogeneous field. We use a simulation based technique to account for the distortion of the ion cloud caused by the inhomogeneous field. In order to demonstrate the gain in kinetic energy resolution compared to conventional 3D Ion Imaging, we use the spatial distribution of H(+) ions emerging from the photodissociation of HCl following the two photon excitation to the V (1)Sigma(+) state. So far a figure of merit of approximately four has been achieved, which means in absolute numbers Delta v/v = 0.022 compared to 0.086 at v approximately = 17,000 m/s. However, this is not a theoretical limit of the technique, but due to our rather short TOF spectrometer (15 cm). The photodissociation of HBr near 243 nm has been used to recognize and eliminate systematic deviations between the simulation and the experimentally observed distribution. The technique has also proven to be essential for the precise measurement of translationally cold distributions.

Intermediate state polarization in multiphoton ionization of HCl.

The paper presents the detailed theoretical description of the intermediate state polarization and photofragment angular distribution in resonance enhanced multiphoton ionization (REMPI) of molecules and the experimental investigation of these effects in the E(1)Sigma(+) and V(1)Sigma(+) states of HCl populated by two-photon transitions. It is shown that the intermediate state polarization can be characterized by the universal parameter b which is in general a complex number containing information about the symmetry of the two-photon excitation and possible phase shifts. The photofragment angular distribution produced by one- or multiphoton excitation of the polarized intermediate state is presented as a product of the intermediate state axis spatial distribution and the angular distribution of the photofragments from an unpolarized intermediate state. Experiments have been carried out by two complementary methods: REMPI absorption spectroscopy of rotationally resolved (E,v'=0<--X,v"=0) and (V,v'=12<--X,v"=0) transitions and REMPI via the Q(0) and Q(1) rotational transitions followed by three-dimensional ion imaging detection. The values of the parameter b determined from experiment manifest the mostly perpendicular nature of the initial two-photon transition. The experimentally obtained H(+) -ion fragment angular distributions produced via the Q(1) rotational transition show good agreement with theoretical prediction.

Photoionization and photodissociation of HCl(B 1Sigma+, J=0) near 236 and 239 nm using three-dimensional ion imaging.

The electronically excited states HCl(*)(E,upsilon(')=0,J(')=0) and HCl(*)(V,upsilon(')=12,J(')=0) have been prepared by two-photon resonant absorption of ground state HCl via Q(0) transitions at 238.719 and at 236.000 nm, respectively. The consequent one-or two-photon excitation at the same wavelength results in the production of H(+), Cl(+), and HCl(+) ions. The speed distributions and anisotropy parameters beta for these ions have been determined by three-dimensional photo-fragment ion imaging based on a position-sensitive delay-line anode assembly. Several results are presented: first, we measured velocity (speed and angle) distributions for HCl(+) due to the electron recoil in the photoionization of HCl(*). Such distributions give information on the photoionization process and on the vibrational distribution of HCl(+) after the laser pulse. Second, the measured beta parameters for Cl(+) and H(+) distributions give information on the symmetries of the upper states in the one-photon photoexcitation of HCl(*). Third, the measured speed distributions for H(+) help to understand the mechanism of the photodissociation of HCl(+) ions.