M Stankiewicz

Dissociative photoionisation of molecules probed by triple coincidence ; double time-of-flight techniques

Two novel experiments producing three-dimensional histograms of time-of-flight correlations have recently been performed at the Daresbury Synchrotron Radiation Source. In the first, the two photoions from the double ionisation of SF6 were detected; in the second, the photoelectron and the O+ ion from the predissociation of O2+. This technique allows a visual insight into the details of dissociative photoionisation which has not been available previously.

The double photoionisation of NH3 using the triple coincidence (PEPIPICO) technique

A simple development of the PEPIPICO technique, utilising the continuum nature of synchrotron radiation, has been used to determine thresholds for fragmentation of NH3 into the channels NH+2+H+ (35.7 and 39.6 eV), NH++H+ and NH++H+2 (43.3 eV) and N++H+ (49.2 eV). The threshold of NH2+3 metastable ion production has also been determined (33.7 eV).

Double photoionisation of methane studied by the triple coincidence (PEPIPICO) technique

Abstract The PEPIPICO technique has been coupled with a source of synchrotron radiation in order to determine thresholds for fragmentation of CH 4 into CH 3 + + H + , CH 2 + + H + , CH + + H + , C + + H + and CH 2 + + H 2 + . The thresholds appear to correlate well with previously calculated electronically excited states of CH 4 2+ .

An energy resolved electron?ion coincidence study near the S 2p thresholds of the SF6 molecule

The fragmentation dynamics of the SF6 molecule following the excitations of S 2p electrons into unoccupied molecular orbitals has been studied using the energy-resolved electron–ion coincidence technique. Fragmentation patterns were found to depend on the particular excitation and on the electronic state of the molecular ion. The spectator resonant Auger decay at the 2p → 6a1g resonance induces changes in the ion distributions as compared to direct photoionization. Furthermore, coincidence spectra related to the same Auger structure display different ion abundances at the 2t2g and 4eg shape resonances. Differences were also found in the Auger decay spectra. These findings give further support for the previously suggested many-electron character of the 4eg shape resonance.

Photofragmentation of 2-deoxy-D-ribose molecules in the gas phase

We have measured the synchrotron-induced photofragmentation of isolated 2-deoxy-D-ribose molecules (C(5)H(10)O(4)) at four photon energies, namely, 23.0, 15.7, 14.6, and 13.8 eV. At all photon energies above the molecules ionization threshold we observe the formation of a large variety of molecular cation fragments, including CH(3) (+), OH(+), H(3)O(+), C(2)H(3) (+), C(2)H(4) (+), CH(x)O(+) (x=1,2,3), C(2)H(x)O(+) (x=1-5), C(3)H(x)O(+) (x=3-5), C(2)H(4)O(2) (+), C(3)H(x)O(2) (+) (x=1,2,4-6), C(4)H(5)O(2) (+), C(4)H(x)O(3) (+) (x=6,7), C(5)H(7)O(3) (+), and C(5)H(8)O(3) (+). The formation of these fragments shows a strong propensity of the DNA sugar to dissociate upon absorption of vacuum ultraviolet photons. The yields of particular fragments at various excitation photon energies in the range between 10 and 28 eV are also measured and their appearance thresholds determined. At all photon energies, the most intense relative yield is recorded for the m/q=57 fragment (C(3)H(5)O(+)), whereas a general intensity decrease is observed for all other fragments- relative to the m/q=57 fragment-with decreasing excitation energy. Thus, bond cleavage depends on the photon energy deposited in the molecule. All fragments up to m/q=75 are observed at all photon energies above their respective threshold values. Most notably, several fragmentation products, for example, CH(3) (+), H(3)O(+), C(2)H(4) (+), CH(3)O(+), and C(2)H(5)O(+), involve significant bond rearrangements and nuclear motion during the dissociation time. Multibond fragmentation of the sugar moiety in the sugar-phosphate backbone of DNA results in complex strand lesions and, most likely, in subsequent reactions of the neutral or charged fragments with the surrounding DNA molecules.

Fragmentation of valence electronic states of SF+6 studied with synchrotron radiation

Fragmentation of the seven valence electronic states of SF+6 has been studied in the range 440–810 A by photoionisation mass spectrometry. The ion yield for SF+5 shows two thresholds at the adiabatic ionisation energy of the ground (X 2T1g) and first two excited states (A2T1u and B 2T2u) of SF+6. SF+4 shows a threshold at the third state (C 2Eg) of SF+6, SF+3 shows two thresholds at the fourth and fith states (D 2T2g and E 2T1u), and SF+2 turns on at the sixth state (F 2A1g). Thus in all cases the appearance energy of a fragment ion occurs at the adiabatic ionisation potential of an electronic state of SF+6, and not at the lower-lying thermochemical energy of the fragment ion. Kinetic energy releases have been measured in SF+5, SF+4 and SF+3 at photon energies just above threshold for production of each ion. In the case of SF+5 a substantial fraction of the available energy is converted into translational energy of the fragments. The relative photoionisation yields of SF+5, SF+4 and SF+3 are compared with values expected from the photoelectron branching ratios of electronic states of SF+6. The latter results overestimate the yields of SF+3 and SF+4 relative to SF+5, and possible explanations are discussed.

Attosecond chirp-encoded dynamics of light nuclei

We study the spectral phase of high-order harmonic emission as an observable for probing ultrafast nuclear dynamics after the ionization of a molecule. Using a strong-field approximation theory that includes nuclear dynamics, we relate the harmonic phase to the phase of the overlap integral of the nuclear wavefunctions of the initial neutral molecule and the molecular ion after an attosecond probe delay. We determine experimentally the group delay of the high harmonic emission from D2 and H2 molecules, which allows us to verify the relation between harmonic frequency and the attosecond delay. The small difference in the harmonic phase between H2 and D2 calculated theoretically is consistent with our experimental results.

State selective photon induced formation of triply charged fragments from the core excited OCS molecule

State selective photon induced formation of triply charged fragments from the core excited OCS molecule

Soft x-ray photoionization and fragmentation of SO2 studied by threshold photoelectron-photoion-photoion coincidence (TPEPIPICO) spectroscopy

The photoionization of SO2 has been studied in the photon energy range 160-560 eV using the SRS at Daresbury Laboratory, UK. Threshold photoelectron and total ion-yield spectra are presented in the region of the S 2p and O 1s edges. Ion branching ratios are obtained from threshold photoelectron-photoion coincidence (TPEPICO) data and the detailed dynamics of the fragmentation process determined using the threshold photoelectron-photoion-photoion coincidence (TPEPIPICO) technique. The data are compared with other results at the S 2p edges, and with previous studies of SO2 in the vacuum ultraviolet spectral region.

Energy distributions of ions produced in photodissociation of in the 17 - 34 eV range

Energy distributions of ions from photodissociation of in the 17 - 34 eV energy range have been measured using photoelectron - photoion TOF spectroscopy. A high accuracy in the determination of the kinetic energies of the released fragments allows for a qualitative comparison of the different dissociation processes. In the lower energy region (17 - 20.5 eV) the production of fragments occurs mainly via resonant processes like post-dissociative autoionization (PDI), autoionization to dissociative states of the ion or dissociation from the ion pair state. In the higher energy region the fragmentation most frequently occurs due to direct photoionization to dissociative states of the ion, although the presence of PDI has also been observed. No fragments are observed with kinetic energies higher than 2 eV.