Photoelectron Photoion Coincidence Method Research Articles

Correlation between a photoelectron and a fragment ion in dissociative ionization of ethanol in intense near-infrared laser fields

The two dissociative ionization channels of ethanol (C2H5OH) induced by an intense near-infrared laser pulse (λ ~ 783 nm), C2H5OH → CH2OH(+) + CH3 + e(-) and C2H5OH → C2H5(+) + OH + e(-), are investigated using photoelectron-photoion coincidence method. It is shown that both the electronic ground state and the first electronically excited state of C2H5OH(+) are produced at the moment of photoelectron emission. From the observed correlation between the electronic states of C2H5OH(+) prepared at the moment of photoelectron emission and the kinetic energy release of the fragment ions, it is revealed that C2H5OH(+) prepared in the electronic ground state at the photoelectron emission gains larger internal energy in the end than that prepared in the electronically excited state. The averaged internal energy of C2H5OH(+) just before the dissociation is found to increase when the laser field intensity increases from 9 to 23 TW∕cm(2) and when the laser pulse duration increases from 35 to 800 fs.

Photodissociation of methyl formate in circumstellar environment: stability under soft X-rays

The methyl formate molecule (HCOOCH3) is one of the most abundant molecules observed in hot molecular cores. Together with acetic acid and the sugar glycolaldehyde, these C2H4O2 molecules are the first triad of isomers detected in interstellar clouds. However, the observations point to a high enhancement in the abundance of methyl formate, which can reach up to 100 times the amount of its second abundant isomer, acetic acid. The observational abundance difference between these isomers has been subject of several works, invoking different formation pathways or survival rates. In this work we present an experimental and theoretical study of photoionization and photodissociation processes of HCOOCH3, to point out clues about its survival to high-energy photons observed in some interstellar environments. In order to obtain the most stable C2H4O2 cations and dications dissociation pathways, spectroscopic and computational methods were used. The measurements were taken employing soft X-ray photons with energies around carbon 1s and oxygen 1s resonances, while the calculations were performed at B3LYP/cc-pVDZ level, at the experimental pressure and temperature. Mass spectra of the photoproduced fragments were obtained using photoelectron photoion coincidence method. The main photodissociation channels are in agreement with the most favourable calculated pathways. Photoionization and photodissociation cross-sections were also determined. The results suggest that the observed high abundance of methyl formate could not be attributed to photodissociation processes.

Site-specific fragmentation caused by Si:1s core-level photoionization of F 3SiCH 2CH 2Si(CH 3) 3 vapor

Ionic fragmentation caused by Si:1s photoionization of 1-trifluorosilyl-2-trimethylsilylethane [F 3SiCH 2CH 2Si(CH 3) 3] vapor was studied by the energy-selected photoelectron photoion coincidence method and monochromatized synchrotron radiation. In the 1s photoionization at the Si atom bonded to three F atoms, H + exceeded the other ions in the peak height, and production of SiF 3 + ion seemed to be reduced. On the other hand, the 1s photoionization at the other Si atom bonded to three CH 3 groups enhanced production of H + ion with high kinetic energy. These results suggest that Si:1s photoionization causes site-specific fragmentation.

Cascade decay of atomic magnesium after photoionization with a photoelectron-photoion coincidence method

For the K-shell decay, magnesium is the first element in the periodic table which shows cascading transitions. We investigated the whole cascade of magnesium by using the photoelectron-photoion coincidence technique on the $1s,$ $2s,$ and $2p$ decay. The experimentally determined and calculated decay probabilities for the ${1s}^{\ensuremath{-}1},$ the ${2s}^{\ensuremath{-}1},$ and the ${2p}^{\ensuremath{-}1}$ decay, i.e., the whole cascade, are in good agreement with each other. For the calculation of higher final ionic charge states, it was found that electron correlations have to be taken into account. The fluorescence yields for the K and L shell and the Coster-Kronig factor for the L shell were determined.

A pulsed field ionization photoelectron–photoion coincidence study of the dissociative photoionization process D 2O+ hν→OD ++D+e −

We have examined the titled reaction near its threshold using the pulsed field ionization photoelectron–photoion coincidence method. The measured 0 K threshold (18.220±0.002 eV) has made possible the determination of more precise values for the 0 K bond dissociation energies for D–OD + (5.584±0.002 eV) and D–OD (5.191±0.002 eV) and the 0 K heats of formation ( ΔH 0 f0's) for OD + (308.83±0.05 kcal/mol) and OD (8.38±0.05 kcal/mol). We found that the available energetic data for the OD/OD + and D 2O/D 2O + system are in excellent agreement with those for the OH/OH + and H 2O/H 2O + system after taking into account the zero-point vibration energies of these species.

Si:2p site-specific excitation and fragmentation of bridged trihalosilyl–trimethylsilyl molecules: role of the bridge and final-state effect

To elucidate site-specific phenomena, we experimentally and computationally studied the spectroscopy and dynamics caused by Si:2p core-level photoexcitation of bridged trihalosilyl–trimethylsilyl molecules. We used the photoionization efficiency curve and the photoelectron photoion coincidence method to study the site-specific phenomena in the Si:2p photoexcitation of F 3SiCH 2CH 2CH 2Si(CH 3) 3, F 3SiCHCHSi(CH 3) 3 and Cl 3SiCCSi(CH 3) 3 in the vapor phase. The site-specific excitation was revealed in the photoionization efficiency curves of all the molecules. The site-specific fragmentation is likely to be more evident when the distance between the two Si sites is large. For the site-specific fragmentation to be caused by the Si:2p photoexcitation, there should not be a CC bond between the atomic site of interest and any other near atomic site around which bond dissociation is undesirable. Not only initial-state effect but also final-state effect is likely to contribute to the occurrence of the different chemical shifts between the two Si sites of the bridged trihalosilyl–trimethylsilyl molecules.

A photoelectron–photoion coincidence method for the investigation of decay probabilities after innershell photoionization

The photoelectron–photoion method presented is shown to be very successful for the quantitative investigation of decay probabilities after innershell photoionization. Especially for the determination of K-shell fluorescence yields of light elements, where only scarce data exist up to now, this method opens up new ways. For all elements we present a general approach to determine fluorescence yields. As an example the investigation of the 1 s decay of free sodium atoms is presented.

Simultaneous determination of radiative and nonradiative decay channels in the neonKshell

The photoelectron-photoion coincidence method is shown to be very successful for the quantitative investigation of the ratio of radiative to nonradiative transitions and for the ratio of single to double Auger transitions. These decay probabilities are important parameters for analytical methods as well as for theoretical descriptions. The method is demonstrated for the neon $1s$ decay. Especially the fluorescence yield and the yield for the double Auger process have been determined both experimentally and theoretically.

Angular distribution of fragment ions formed by K-shell excitation of N2 molecule

Angular distributions of the fragment ions produced by the K-shell excitation of N2 molecule have been studied by use of a rotatable time-of-flight (TOF) mass spectrometer. The TOF spectra of N+ and N2+ have been measured at 0, 30, 54.7 and 90' by a photoelectron-photoion coincidence method. Asymmetry parameters obtained for N+ and N2+ are equal. The asymmetry parameters at the photon energies corresponding to the transitions from 1s to the discrete molecular orbitals (2p pi g, 3s sigma g and so forth) are consistent with the values expected from the symmetry of the transition. In the region of the shape resonance (around 419 eV), the values of these parameters are smaller than those obtained by theoretical prediction.

Dissociation processes of O2in the VUV region 500-700 AA

Predissociation from the b4 Sigma g-, B2 Sigma g- and c4 Sigma u- states of O2+ were observed and analysed by using the threshold photoelectron-photoion coincidence method and monochromatic synchrotron radiation as an excitation source. It is found that these three states predissociate to the dissociation limits via many channels. Branching ratios for the dissociation limits from the B2 Sigma g- and c4 Sigma u- states and yield spectra of O2+ and O+ in coincidence with threshold electrons are discussed. The results indicate that the vibrational levels of the O2+ bound states above the dissociation limit predissociate considerably and that the v'=4 level of the B2 Sigma g- state of O2+ shows abnormal behaviour. It is evident that the dissociation process of the v'-0 level of the c4 Sigma u- state of O2+ is different from that of v'=1 level.

O+2 ions dissociation studied by threshold photoelectron–photoion coincidence method

The threshold photoelectron–photoion coincidence method was used to study O+2 dissociation. Previous results were confirmed and a careful analysis of the time of flight peak shapes using a Monte Carlo simulation gave us new results. The lifetime of the B 2Σ−g state was measured to be 70∓25 ns independently of the vibrational quantum number. The III 2∏u state which dissociates towards the O+(2D0)+O(3P) and O+(2P0)+O(3P) limits shows anisotropic distribution of fragment ions. The c 4Σ−u(v=0) state which was previously observed to dissociate into O+(4S0)+O(1D), its adiabatic limit, is seen to predissociate also about 40% towards the ground state limit.