Cations of halogenated methanes: adiabatic ionization energies, potential energy surfaces, and ion fragment appearance energies

We compare characteristics of electron impact ionization (EI) and multiphoton ionization (MPI) of doped superfluid helium droplets using the same droplet source. Selected dopant ion fragments from the two ionization schemes demonstrate different dependence on the doping pressure, which could be attributed to the different ionization mechanisms. While EI directly ionizes helium atoms in a droplet therefore has higher yields for bigger droplets (within a limited size range), MPI is insensitive to the helium in a droplet and is only dependent on the number of dopant molecules. The optimal timing of the ionization pulse also varies with the doping pressure, implying a velocity slip among different sized droplets. Calculations of the doping statistics and ionization probabilities qualitatively agree with the experimental data. Our results offer a word of caution in interpreting the pressure and timing dependence of superfluid helium droplets, and we also devise a scheme in achieving a high degree of doping while limiting the contribution of dopant clusters.

Section: Methodsmentioning

confidence: 97%

Electron impact ionization and multiphoton ionization of doped superfluid helium droplets: A comparison

Zhang

Kong

2016

“…Complexes between CHF + (fluorocarbene cation) and HX (X = F, Cl, Br), which have the overall stoichiometry CH 2 FX + , are well known in theoretical studies. 11 The complex [CHF• • • HF] + has C 1 symmetry and is structurally very different from the X 1 A 1 ground state of CH 2 F 2 . It is claimed 11 to be more stable than the corresponding C 2V isomer by 929 cm 1 .…”

Section: Introductionmentioning

confidence: 99%

“…11 The complex [CHF• • • HF] + has C 1 symmetry and is structurally very different from the X 1 A 1 ground state of CH 2 F 2 . It is claimed 11 to be more stable than the corresponding C 2V isomer by 929 cm 1 . In the light of these complexes, Forysinski et al considered 4 the possibility that ionization of CH 2 F 2 does not lead to states exhibiting C 2V symmetry, but the Franck-Condon factors (FCFs) for these complexes were too low for such species to appear in their Pulsed-Field-Ionization Zero-Kinetic-Energy (PFI-ZEKE) spectrum.…”

Section: Introductionmentioning

confidence: 99%

The ionic states of difluoromethane: A reappraisal of the low energy photoelectron spectrum including ab initio configuration interaction computations

Palmer

Biczysko

Baiardi

et al. 2017

A new synchrotron-based study of the photoelectron spectrum (PES) of difluoromethane is interpreted by an ab initio analysis of the ionic states, which includes Franck-Condon (FC) factors. Double differentiation of the spectrum leads to significant spectral sharpening; the vibrational structure observed is now measured with greater accuracy than in previous studies. Several electronic structure methods are used, including equation of motion coupled cluster calculations with single and double excitations (EOM-CCSD), its ionization potential variant EOM-IP-CCSD, 4th order Møller-Plesset perturbation theory (MP4SDQ) configuration interaction (CI), and complete active space self-consistent-field (CASSCF) methods. The adiabatic ionization energies (AIEs) confirm the assignments as band I, one state 1B (12.671 eV); band II, three states, 1B (14.259) < 1A (15.030) < 1A (15.478 eV); and band III, three states, 2B (18.055) < 2A (18.257) < 2B (18.808 eV). The three ionizations in each of the bands II and III lead to selective line broadening of the PES structure, which is attributed to vibronic overlap. The apparent lack of a vibrational structure attributable to both the 1A and 2A states in the PES arises from line broadening with the preceding states 1B and 2B, respectively. Although these A states clearly overlap with their adjacent higher IE, some vibrational structure is observed on the higher IE. The effects of vibronic coupling are evident since the observed structure does not fit closely with the calculated Born-Oppenheimer FC profiles. Correlation of the lowest group of four AIEs in the PES of other members of the CHX group, where X = F, Cl, Br, and I, clearly indicate these effects are more general.

“…It is interesting to note in Fig. 6 that, if we only consider the range of n 2 + values experimentally observed (10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26), the ν 2 + spacing ( G(n 2 + )) is roughly constant and we could think that the anharmonicity is negligible, which is not the case. Hence, we cannot use a simple harmonic model to directly derive IE ad.…”

Section: F the Adiabatic Ionization Potential Of Cfmentioning

confidence: 95%

“…[15][16][17][18][19][20][21][22][23] Such an interest is in part explained by the geometrical features of the CF 3 • /CF 3 + system: while the CF 3…”

Section: Introductionmentioning

confidence: 99%

Comprehensive vacuum ultraviolet photoionization study of the CF3• trifluoromethyl radical using synchrotron radiation

Dossmann

Garcı́a

Nahon

et al. 2012

The trifluoromethyl radical, CF(3)(●), is studied for the first time by means of threshold photoelectron spectroscopy (TPES). The radical is produced in the gas phase using the flash-pyrolysis technique from hexafluoroethane as a precursor. CF(3)(+) total ion yield and mass-selected TPES of the radical are recorded using a spectrometer based upon velocity map imaging and Wiley-McLaren time-of-flight coupled to the synchrotron radiation. The high resolution of the instrument and of the photons allows the observation of rich vibrational progressions in the TPES of CF(3)(●). By using Franck-Condon factors computed by Bowman and coworkers, we have been able to simulate the TPES. The initial vibrational temperature of the radical beam has been evaluated at 350 ± 70 K. The structures have been identified as transitions between (n(1),n(2)) and (n(1)(+),n(2)(+)) vibrational levels of CF(3) and CF(3)(+) with small excitation of the breathing mode, ν(1)(+) (,) and large excitation (n(2)(+) = 10-26) of the umbrella mode, ν(2)(+), in the cation. From the energy separation between the two resolved peaks of each band, a value of 994 ± 16 cm(-1) has been derived for the ν(1)(+) breathing frequency of CF(3)(+). For the high-lying n(2)(+) levels, the apparent ν(2)(+) umbrella spacing, 820 ± 14 cm(-1), is fairly constant. Taking into account the ν(2)(+) anharmonicity calculated by Bowman and coworkers, we have deduced ν(2)(+) = 809 ± 14 cm(-1), and semi-empirical estimations of the adiabatic ionization energy IE(ad.)(CF(3)(●)) are proposed in good agreement with most of previous works. A value of the vertical ionization potential, IE(vert.)(CF(3)(●)) = 11.02 eV, has been derived from the observation of a photoelectron spectrum recorded at a fixed photon energy of 12 eV.