Maurice H. M. Janssen

A photoelectron-photoion coincidence imaging apparatus for femtosecond time-resolved molecular dynamics with electron time-of-flight resolution of sigma=18 ps and energy resolution Delta E/E=3.5%

We report on the construction and performance of a novel photoelectron-photoion coincidence machine in our laboratory in Amsterdam to measure the full three-dimensional momentum distribution of correlated electrons and ions in femtosecond time-resolved molecular beam experiments. We implemented sets of open electron and ion lenses to time stretch and velocity map the charged particles. Time switched voltages are operated on the particle lenses to enable optimal electric field strengths for velocity map focusing conditions of electrons and ions separately. The position and time sensitive detectors employ microchannel plates (MCPs) in front of delay line detectors. A special effort was made to obtain the time-of-flight (TOF) of the electrons at high temporal resolution using small pore (5 microm) MCPs and implementing fast timing electronics. We measured the TOF distribution of the electrons under our typical coincidence field strengths with a temporal resolution down to sigma=18 ps. We observed that our electron coincidence detector has a timing resolution better than sigma=16 ps, which is mainly determined by the residual transit time spread of the MCPs. The typical electron energy resolution appears to be nearly laser bandwidth limited with a relative resolution of DeltaE(FWHM)/E=3.5% for electrons with kinetic energy near 2 eV. The mass resolution of the ion detector for ions measured in coincidence with electrons is about Deltam(FWHM)/m=14150. The velocity map focusing of our extended source volume of particles, due to the overlap of the molecular beam with the laser beams, results in a parent ion spot on our detector focused down to sigma=115 microm.

Production of maximally aligned O(1D) atoms from two-step photodissociation of molecular oxygen

Two-step photodissociation of O2 with a total excitation energy of 7.7 eV yields O(1D) atoms with their angular momenta aligned perpendicular to the fragment recoil axis. Correlation rules linking the bound molecule with the separated atoms predict such strong alignment only for an adiabatic process.

A short pulse (7 μs FWHM) and high repetition rate (dc-5kHz) cantilever piezovalve for pulsed atomic and molecular beams

In this paper we report on the design and operation of a novel piezovalve for the production of short pulsed atomic or molecular beams. The high speed valve operates on the principle of a cantilever piezo. The only moving part, besides the cantilever piezo itself, is a very small O-ring that forms the vacuum seal. The valve can operate continuous (dc) and in pulsed mode with the same drive electronics. Pulsed operation has been tested at repetition frequencies up to 5 kHz. The static deflection of the cantilever, as mounted in the valve body, was measured as a function of driving field strength with a confocal microscope. The deflection and high speed dynamical response of the cantilever can be easily changed and optimized for a particular nozzle diameter or repetition rate by a simple adjustment of the free cantilever length. Pulsed molecular beams with a full width at half maximum pulse width as low as 7 micros have been measured at a position 10 cm downstream of the nozzle exit. This represents a gas pulse with a length of only 10 mm making it well matched to for instance experiments using laser beams. Such a short pulse with 6 bar backing pressure behind a 150 microm nozzle releases about 10(16) particles/pulse and the beam brightness was estimated to be 4x10(22) particles/(s str). The short pulses of the cantilever piezovalve result in a much reduced gas load in the vacuum system. We demonstrate operation of the pulsed valve with skimmer in a single vacuum chamber pumped by a 520 l/s turbomolecular pump maintaining a pressure of 5x10(-6) Torr, which is an excellent vacuum to have the strong and cold skimmed molecular beam interact with laser beams only 10 cm downstream of the nozzle to do velocity map slice imaging with a microchannel-plate imaging detector in a single chamber. The piezovalve produces cold and narrow (Delta v/v=2%-3%) velocity distributions of molecules seeded in helium or neon at modest backing pressures of only 6 bar. The low gas load of the cantilever valve makes it possible to design very compact single chamber molecular beam machines with high quality cold and intense supersonic beams. The high speed cantilever piezovalve may find broad applicability in experiments where short and strong gas pulses are needed with only modest pumping, the effective use of (expensive) samples, or the production of cold atomic and molecular beams.

Femtosecond velocity map imaging of dissociative ionization dynamics in CF3IPresented at the Stereodynamics 2000 Conference on Dynamics and Stereodynamics of Chemical Reactions, El Escorial, Madrid, December 1–5, 2000.

The multi-photon dissociation dynamics of CF3I has been studied with femtosecond pump–probe spectroscopy and velocity map ion imaging. The CF3+ and I+ fragments produced in a time-delayed pump–probe excitation are detected with a two-dimensional ion imaging setup in a velocity map imaging configuration. The ion images for the selected ionic photofragments provide the velocity and angular distribution of the recoiling fragments. The experiments were performed with both parallel and perpendicular polarization geometry of the pump laser, at 264 nm, versus the probe laser at 396 nm. The velocity and angular distributions provide information on the multi-photon pathways and the potential energy surfaces involved. The CF3+ fragments are mainly formed by a two-photon pump excitation at 264 nm, via the one-photon resonant A band, to the 5pπ7sσ(2Π1/2) Rydberg state followed by a one-photon probe excitation to the CF3I+ parent states with subsequent dissociation. Analysis of the I+ data indicates that at most delay times the fragments are formed via a two-photon absorption at 264 nm to the Rydberg state, followed by a two-photon transition at 396 nm to the state of the parent ion. However, at delay times around 200–400 fs the kinetic energy distribution of the I+ fragments changes dramatically relative to 0 fs and 1000 fs. The origin of the very slow I+ fragments is probably a bound–free–bound excitation via the repulsive A band to a higher lying ion-pair state. It is shown that the ion imaging technique combined with femtosecond time-resolved spectroscopy provides a direct view of the complex dynamics and multi-photon pathways involved in the dissociative photodynamics of CF3I.

Two-dimensional imaging of photofragments

The technique of photofragment imaging is described, and several examples of the power of the technique are presented. Two-dimensional images of state-selected photofragments from the photodissociations of CD3I and H2S illustrate how photofragment imaging reveals β parameters, brancing ratios, Doppler profiles and vector correlations. Comparisons are made with Doppler profiling and one-dimensional time-of-flight techniques.

Imaging photoelectron circular dichroism of chiral molecules by femtosecond multiphoton coincidence detection

Here, we provide a detailed account of novel experiments employing electron-ion coincidence imaging to discriminate chiral molecules. The full three-dimensional angular scattering distribution of electrons is measured after photoexcitation with either left or right circular polarized light. The experiment is performed using a simplified photoelectron-photoion coincidence imaging setup employing only a single particle imaging detector. Results are reported applying this technique to enantiomers of the chiral molecule camphor after three-photon ionization by circularly polarized femtosecond laser pulses at 400 nm and 380 nm. The electron-ion coincidence imaging provides the photoelectron spectrum of mass-selected ions that are observed in the time-of-flight mass spectra. The coincident photoelectron spectra of the parent camphor ion and the various fragment ions are the same, so it can be concluded that fragmentation of camphor happens after ionization. We discuss the forward-backward asymmetry in the photoelectron angular distribution which is expressed in Legendre polynomials with moments up to order six. Furthermore, we present a method, similar to one-photon electron circular dichroism, to quantify the strength of the chiral electron asymmetry in a single parameter. The circular dichroism in the photoelectron angular distribution of camphor is measured to be 8% at 400 nm. The electron circular dichroism using femtosecond multiphoton excitation is of opposite sign and about 60% larger than the electron dichroism observed before in near-threshold one-photon ionization with synchrotron excitation. We interpret our multiphoton ionization as being resonant at the two-photon level with the 3s and 3p Rydberg states of camphor. Theoretical calculations are presented that model the photoelectron angular distribution from a prealigned camphor molecule using density functional theory and continuum multiple scattering X alpha photoelectron scattering calculations. Qualitative agreement is observed between the experimental results and the theoretical calculations of the Legendre moments representing the angular distribution for the two enantiomers. The electron-ion coincidence technique using multiphoton ionization opens new directions in table-top analytical mass-spectrometric applications of mixtures of chiral molecules.

State-to-state photodissociation of OCS (nu(2)=0,1 vertical bar JlM). I. The angular recoil distribution of CO (X (1)Sigma(+);v=0 vertical bar J)

State-to-state photodissociation experiments of OCS at 230 nm are reported using hexapole state selection of the parent molecule and velocity map ion imaging of the angular recoil of the CO photofragment. The role of the initial rovibrational state (ν2=0,1|JlM) of OCS on the angular recoil distribution is investigated. The CO (X 1Σ+;v=0|J) rotational distribution as well as the angular recoil anisotropy parameter β of the CO photofragment are reported for dissociation of single rovibrational (ν2=0,1|JlM) quantum states of OCS. A strong dependence of the anisotropy parameter β on the initial bending state, ν2=0 or 1, of OCS is observed. The effects of the initial bending state of OCS are rationalized in terms of the strong angular dependence of the transition dipole moment function of OCS for the 1 1Σ−(1 1A″) and 1 1Δ(2 1A′) excited state surfaces involved in the dissociation at 230 nm. The state-to-state imaging experiment provides a revised and improved determination of the binding energy of OCS (ν1,ν2,ν...

The correlation between 1(P-2(3/2))/I(P-2(1/2)) branching and CH3 rotation in photolysis of single quantum state-selected CH3I (JK=11)

Abstract The dependence of the I ( 2 P 3/2 )/ I ( 2 P 1/2 ) branching ratio on the end-over-end rotational state of the methyl photofragment, CH 3 (ν 2 =2|N,K=1) , after photodissociation at 266 nm of single quantum state-selected CH 3 I (JK=11) molecules is reported. Using velocity map imaging the spatial velocity distribution of the vibrationally excited CH 3 ( ν 2 =2) photofragment was detected. The I ( 2 P 3/2 ) and I ( 2 P 1/2 ) yields were extracted from the speed distribution of rotationally selected CH 3 (ν 2 =2|N,K=1) photofragments. Quantitative analysis shows a strongly increasing I ( 2 P 3/2 )/ I ( 2 P 1/2 ) branching ratio with increasing CH 3 end-over-end rotational motion, in reasonable agreement with recent three-dimensional state-to-state ab initio calculations.

Photodissociation of laboratory oriented molecules: Revealing molecular frame properties of nonaxial recoil

We report the photodissociation of laboratory oriented OCS molecules. A molecular beam of OCS molecules is hexapole state-selected and spatially oriented in the electric field of a velocity map imaging lens. The oriented OCS molecules are dissociated at 230 nm with the linear polarization set at 45 degrees to the orientation direction of the OCS molecules. The CO(nu=0,J) photofragments are quantum state-selectively ionized by the same 230 nm pulse and the angular distribution is measured using the velocity map imaging technique. The observed CO(nu=0,J) images are strongly asymmetric and the degree of asymmetry varies with the CO rotational state J. From the observed asymmetry in the laboratory frame we can directly extract the molecular frame angles between the final photofragment recoil velocity and the permanent dipole moment and the transition dipole moment. The data for CO fragments with high rotational excitation reveal that the dissociation dynamics is highly nonaxial, even though conventional wisdom suggests that the nearly limiting beta parameter results from fast axial recoil dynamics. From our data we can extract the relative contribution of parallel and perpendicular transitions at 230 nm excitation.

Velocity map photoelectron-photoion coincidence imaging on a single detector

Here we report on a new simplified setup for velocity map photoelectron-photoion coincidence imaging using only a single particle detector. We show that both photoelectrons and photoions can be extracted toward the same micro-channel-plate delay line detector by fast switching of the high voltages on the ion optics. This single detector setup retains essentially all the features of a standard two-detector coincidence imaging setup, viz., the high spatial resolution for electron and ion imaging, while only slightly decreasing the ion time-of-flight mass resolution. The new setup paves the way to a significant cost reduction in building a coincidence imaging setup for experiments aiming to obtain the complete correlated three-dimensional momentum distribution of electrons and ions.