Chemical Dynamics Beamline Research Articles

The dynamics of hydrogen abstraction reactions: Crossed-beam reaction Cl+n-C5H12→C5H11+HCl

We present a crossed molecular beam study of the dynamics of the metathesis reaction Cl+n-C5H12→HCl+C5H11 at a collision energy of 16.8 kcal/mol. The experiments were conducted on the Chemical Dynamics Beamline at the Advanced Light Source, using tunable undulator radiation to effect soft ionization of the pentyl radical product. Laboratory angular distributions and time-of-flight spectra at many laboratory angles were used to obtain the center-of-mass translational energy and angular distributions. These distributions were found to be strongly coupled, with the forward scattered pentyl radical formed extremely cold, while the backscattered radicals were formed leaving nearly 15 kcal/mol in internal energy in the products. These results are contrasted with recent studies on the analogous reaction with propane, suggesting in this case direct involvement of the carbon skeleton in the collision process.

Photodissociation of acrylonitrile at 193 nm: A photofragment translational spectroscopy study using synchrotron radiation for product photoionization

We have investigated the photodissociation of acrylonitrile (H2CCHCN) at 193 nm using the technique of photofragment translational spectroscopy. The experiments were performed at the Chemical Dynamics Beamline at the Advanced Light Source and used tunable vacuum ultraviolet synchrotron radiation for product photoionization. We have identified four primary dissociation channels including atomic and molecular hydrogen elimination, HCN elimination, and CN elimination. There is significant evidence that all of the dissociation channels occur on the ground electronic surface following internal conversion from the initially optically prepared state. The product translational energy distributions reflect near statistical simple bond rupture for the radical dissociation channels, while substantial recombination barriers mediate the translational energy release for the two molecular elimination channels. Photoionization onsets have provided additional insight into the chemical identities of the products and their internal energy content.

Primary and secondary processes in the 193 nm photodissociation of vinyl chloride

We have investigated the photodissociation of vinyl chloride (H2CCHCl) at 193 nm using the technique of photofragment translational spectroscopy. The experiments were performed at the Chemical Dynamics Beamline at the Advanced Light Source and used vacuum ultraviolet synchrotron radiation for product photoionization. We have observed five primary dissociation channels following an initial π*←π excitation. The majority of Cl atoms originate from an excited-state dissociation. The remaining dissociation channels are consistent with competition on the ground electronic state following internal conversion from the optically prepared state. These channels include atomic and molecular hydrogen elimination, HCl elimination, and a translationally slow Cl elimination channel. We have also identified and characterized two secondary decomposition channels: (1) the elimination of Cl from chlorovinyl radicals following the primary atomic hydrogen elimination channel, and (2) hydrogen atom elimination from vinyl radicals following the primary atomic Cl elimination. By measuring the truncation in the translational energy distribution for C2H2Cl products from primary atomic hydrogen elimination we deduce a barrier for the reverse reaction of Cl+acetylene of 11±2 kcal/mol. Since Cl is known to add rapidly to acetylene with no activation barrier, we conclude that H loss primarily forms the ClCCH2 isomer, and that the observed 11 kcal/mol barrier pertains to a concerted addition/rearrangement path to form the α-chlorovinyl radical. Finally, we report low-resolution photoionization spectra for the nascent vinyl radical and HCl photoproducts, in which redshifts in the ionization onsets can be related to the internal energy content.

A high resolution pulsed field ionization photoelectron study of O2 using third generation undulator synchrotron radiation

We have improved a newly developed experimental scheme for high resolution pulsed field ionization photoelectron (PFI-PE) studies [Hsu et al., Rev. Sci. Instrum. (in press)] using the high resolution monochromatized multibunch undulator synchrotron source of the Chemical Dynamics Beamline at the Advanced Light Source. This improved scheme makes possible PFI-PE measurements with essentially no contamination by background electrons arising from direct photoionization and prompt autoionization processes. We present here a preliminary analysis of the rotationally resolved PFI-PE spectrum for O2 obtained at a resolution of 0.5 meV (full-width-at-half-maximum) in the photon energy range of 18.1–19.4 eV, yielding accurate ionization energies for the transitions O2+(b 4Σg−, v+=0–9, N+=1)←O2(X 3Σg−, v=0, N=1).

High resolution threshold and pulsed field ionization photoelectron spectroscopy using multi-bunch synchrotron radiation

We have demonstrated a resolution of 0.8 meV [full width at half-maximum (FWHM)] for threshold photoelectron measurements using a steradiancy-type zero kinetic energy photoelectron (ZEKE-PE) analyzer and the high resolution monochromatized vacuum ultraviolet (VUV) undulator synchrotron radiation of the chemical dynamics beamline at the advanced light source (ALS). Using this high resolution ZEKE-PE energy analyzer to filter prompt electrons and by employing a proper voltage pulsing scheme adapted to the timing structure of the ALS, we have achieved a resolution of 0.5 meV (FWHM) for pulsed field ionization photoelectron (PFI-PE) measurements with little contamination from prompt photoelectrons produced from direct photoionization and autoionizing processes. The experiment scheme presented here is generally applicable to PFI-PE studies using multi-bunch VUV synchrotron radiation at other synchrotron radiation facilities.

A high resolution energy-selected kinetic energy release study of the process SF6+hν→SF5++F+e−: Heat of formation of SF5+

Using the newly constructed photoelectron-photoion coincidence apparatus associated with the chemical dynamics beamline at the advanced light source, we have performed a high resolution energy-selected kinetic energy release measurement for the dissociative photoionization process SF6+hν→SF5++F+e−. After taking into account the center-of-mass kinetic energy release, the thermochemical threshold for this process is determined to be 14.11±0.08 eV. This value yields 18.5±1.9 and −202.9±2.2 kcal/mol for the heats of formation at 0 K for SF5+ and SF5, respectively.

Unraveling the dissociation of dimethyl sulfoxide following absorption at 193 nm

We have studied the photodissociation of dimethyl sulfoxide, DMSO-h6 and DMSO-d6, at 193 nm using the technique of photofragment translational spectroscopy with a tunable vacuum ultraviolet product probe provided by undulator radiation on the Chemical Dynamics Beamline at the Advanced Light Source. In contrast to previous investigations we have found the dissociation to proceed via a stepwise mechanism involving multiple reaction channels. The primary dissociation, S–C bond cleavage to eliminate a methyl radical, was found to have two competing channels with distinct translational energy distributions. The translational energy distribution for the major primary dissociation channel suggests that it proceeds in a statistical manner on the ground electronic surface following internal conversion. In competition with this channel is a primary dissociation that exhibits a translational energy distribution suggestive of dissociation on an excited electronic surface with most of the available energy partitioned into translational and electronic degrees of freedom. Secondary decomposition of the CD3SO intermediate was found to proceed exclusively via C–S bond cleavage, CD3SO→CD3+SO. However, secondary decomposition of the CH3SO intermediate was found to exhibit competition between CH3SO→CH3+SO and CH3SO→CH2SO+H. The dissociation to CH3 and SO was the major secondary decomposition channel with the translational energy distribution indicating a barrier to recombination of >8 kcal/mol. While a minor hydrogen atom elimination channel was found to play a role in secondary decomposition of CH3SO intermediates, no analogous secondary C–D bond cleavage was detected from the CD3SO intermediates indicating the importance of tunneling in the secondary decomposition of CH3SO.

A high resolution photoionization study of Ne and Ar: Observation of mass analyzed threshold ions using synchrotron radiation and direct current electric fields

Using the high resolution vacuum ultraviolet (vuv) photon source provided by the monochromatized undulator synchrotron radiation of the Chemical Dynamics Beamline at the Advanced Light Source, we have measured the photoionization efficiency (PIE) spectrum for Ne in the energy range of 21.56–21.67 eV at a wavelength resolution of 0.3 meV [full width at half-maximum (FWHM)]. The PIE spectra for Ne obtained using 0.76 and 2.4 V/cm electric fields reveal autoionizing features attributable to the Rydberg states Ne[2p5ns′(1/2)1; n=14–29] and Ne[2p5nd′(3/2)1; n=12–35] converging to the spin–orbit excited Ne+(2P1/2) state. The positions of these Rydberg states are compared to previous experimental results and those calculated using the quantum defects and IE for Ne+(2P1/2) given in Moore [Natl. Stand Ref. Data Ser. Natl. Bur. Stand. 35 (1971)]. We have also observed mass analyzed threshold ions (MATI) for Ne formed in the Ne+(2P3/2,1/2) states. For Ar, only the MATI peak for Ar+(2P3/2) is observed. The failure to observe the MATI peak for Ar+(2P1/2) is attributed to shorter lifetimes of high-n Ar[3p5ns′(1/2)1] and Ar[3p5nd′(3/2)1] Rydberg states compared to the minimum time required for separating the prompt Ar+ ions from the field ionized Ar+ ions in this experiment. The MATI peaks for Ne+(2P3/2,1/2) achieves a resolution of 0.7 meV (FWHM). These MATI spectra for Ne and Ar reported here represent the first such studies made using a cw vuv light source and dc electric fields.

A differentially pumped harmonic filter on the Chemical Dynamics Beamline at the Advanced Light Source

A differentially pumped rare gas cell has been developed to suppress undulator harmonics on the Chemical Dynamics Beamline at the Advanced Light Source. Greater than 104 suppression of the harmonics has been demonstrated with no measurable (<5%) attenuation of the fundamental. The overall design is presented, and vacuum and optical performance are reported.