Vacuum UV Photoabsorption Spectrum Research Articles

The vacuum UV photoabsorption spectrum of 1,1-dibromo-2,2-difluoroethylene (1,1-Br2C2F2) in the 5-15 eV photon energy range. Combining experimental and theoretical investigations

Using synchrotron radiation, we have measured the vacuum UV photoabsorption spectrum (PAS) of 1,1-Br2C2F2 for the first time between 5 eV and 15 eV photon energy. Quantum chemical calculations have been performed for the determination of excitation and ionization energies. At low energy a weak broad band spreads between 5.2 eV and 6.0 eV and is assigned to nBr→Rpz (31B2) valence transition. A broad continuum starting at 6.18 eV corresponds to the π(3b1)→5s (41B1) Rydberg transition. Starting at Eexcad=6.595 eV a group of four narrower peaks likely corresponds to the vibrational excitation of the characteristic π(3b1)→π*(71A1) valence transition. The features evenly spaced by 1315 cm−1 are assigned to the C=C stretching vibration. Between 7.0 eV and 11.5 eV, several weak or very strong sharp features are observed. The 3b1→5p Rydberg transition is characterized by a short progression starting at 7.208 eV. A longer progression, assigned to 3b1→4d Rydberg transition, is observed up from 7.932 eV. Both Rydberg states converge to the X˜2B1 ionic ground state calculated at IEad=9.40 eV with quantum defects δ=2.51 and δ=0.96 respectively. The vibrational analysis provides vibrational wavenumbers ω1=1525±20 cm−1, ω2=975±40 cm−1, ω3=620±40 cm−1, ω4=355±20 cm−1 and ω5=185±30 cm−1 averaged over the 5p and 4d Rydberg states. An interaction has likely to be considered between the 6s and 4d Rydberg states. Above 11.5 eV, several strong sharp and broad bands are tentatively assigned to transitions to Rydberg states converging to excited ionic states of 1,1-Br2C2F2+. For one of these a vibrational structure is observed and a tentative assignment is proposed.

Vacuum UV photoabsorption spectroscopic and photoionization mass spectrometric study of geminal dibromoethylene (1,1-Br2C2H2) in the 5–15eV range. Experiment and theory

The vacuum UV photoabsorption spectrum (PAS) of 1,1-C2H2Br2 has been examined between 5 eV and 15 eV using the vacuum UV light of synchrotron radiation. For the first time the photoionization mass spectroscopy and efficiency of 1,1-C2H2Br2+ has been measured in the same energy range. A quantum chemical calculation is proposed for the investigation of the neutral and ionized states. The photoionization efficiency curve (PIEC) of 1,1-C2H2Br2+ provided the IEad(X˜2B1)= 9.617±0.005 eV. A vibrational structure gave ω3+=1336 cm−1 and most likely ω4+=483 cm−1. The PIEC of C2H2Br+ shows the lowest appearance energy to be at 11.14±0.02 eV. The onset at 11.56±0.01 eV likely corresponds to the excitation of Br in the 2P1/2 spin-orbit state. In the vacuum UV-PAS the broad band observed at 6.231 eV includes nσ→Rs, 2b1(π)→π* valence transitions and the 2b1→3 s Rydberg transition in agreement with the present calculations. An isolated weak continuum at 7.15 eV is assigned to the nπ→π* transition. The 2b1→3 s Rydberg transition is characterized by a short progression starting at 6.583 eV. A series of long progressions observed between 7.364 eV and 9.666 eV has been analyzed in terms of vibrational transitions to np- (δ= 0.544) and nd-type (δ= -0.04) Rydberg states all converging to the X˜2B1 ionic ground state. An analysis has been attempted providing average values of the vibrational wavenumbers ω3≈1300 cm−1 (or 162 meV), ω4≈480 cm−1 (or 60 meV) and ω5≈145 cm−1 (or 18 meV). By the same way several other Rydberg states were analyzed. For the first time the vacuum UV spectrum of 1,1-C2H2Br2 has been recorded above 10 eV and up to 15 eV. Several broad bands are tentatively assigned to transitions to Rydberg states converging to excited ionic states of 1,1-C2H2Br2+. For one of these a vibrational structure is observed and a tentative assignment is proposed.

The vacuum UV photoabsorption spectroscopy of the trans-dichloroethylene (1,2-ClHC=CClH) in the 5–20 eV range: Experiment and theory

This paper investigates in detail the vacuum UV photoabsorption spectrum of trans-1,2-C2H2Cl2 in the 5–20 eV range using synchrotron radiation. Quantum chemical calculations are applied to the electronic transitions. Particular attention has been paid to the 2au(π)→2bg(π*) transition included in the broad band observed at 6.30 eV. The Rydberg 2au(π)→R3s transition is presumably observed at 6.27 eV. Below 6.0 eV the structure(s) are assigned to π→σ* transition(s). The abundantly structured spectrum observed between 7.0 eV and 12.0 eV has been analyzed in terms of vibronic transitions to ns- (δ¯=0.844) and nd/4f- (δ¯=0.03/0.011) Rydberg states, converging to the 2au−1 ionization continuum, to two series of np- (δ¯= 0.66 and 0.40) states converging to the 5ag−1 continuum, to ns- (δ¯=0.983) and nd- (δ¯= –0.06) states converging to the 4bu−1 continuum and to two np-type (δ¯= 0.51 and 0.26) series converging to the 1bg−1 continuum. These data are complemented by an analysis of the vibrational structure of the individual Rydberg states. Comparisons with previously reported values are provided. Several other spectral features Table S2 (see Supplementary Material) in the 12–20 eV range are tentatively assigned to transitions to Rydberg states converging to higher ionic excited states of trans-1,2-C2H2Cl2. Assignments of the vibrational structure of some of these transitions are also proposed.

The vacuum UV photoabsorption spectrum of the geminal dichloroethylene (1,1-C2H2Cl2) in the 5–20 eV range. A vibrational analysis of the valence and Rydberg states

The vacuum UV photoabsorption spectrum of 1,1-C2H2Cl2 is presented and discussed in detail in the 6–20 eV photon energy range. The broad band observed at 6.488 eV includes the 2b1(π) → σ*, 2b1(π) → π* valence transitions and the 2b1(π) → 3s Rydberg transitions. A quantum chemical calculation analysis is proposed and applied to the intertwined vibrational structure belonging to these transitions. For the π → σ* and π → π* transitions short vibrational progressions are observed, analyzed and tentatively assigned. The π → 3s Rydberg transition is characterized by a single progression starting at 6.746 eV. The fine structure observed between 7.5 and 10.1 eV has been analyzed in terms of vibronic transitions to ns- (δ = 0.89), np- (δ = 0.59 and 0.37) and nd-type (δ = 0.16) Rydberg states all converging to the 1,1-C2H2Cl2+(2B1) ionic ground state. The vibrational structure analysis excitation leads to the following wavenumbers: ω2 ≈ 1310 cm−1 (162 meV), ω4 ≈ 650 cm−1 (81 meV) and ω5 ≈ 290 cm−1 (37 meV). These modes can be assigned to the C=C stretching, to the symmetric C–Cl stretching and to the symmetric Cl–C–Cl bending vibrations respectively. By the same way, eight other Rydberg states were analyzed. For the first time the vacuum UV spectrum of 1,1-C2H2Cl2 has been recorded in the 10–20 eV range, revealing intense broad bands which are assigned to transitions to Rydberg states converging to excited states of 1,1-C2H2Cl2+. Assignments are also proposed for the vibrational excitations observed in this range.

The vacuum UV photoabsorption spectroscopy of the geminal ethylene difluoride (1,1-C2H2F2). The vibrational structure and its analysis

The vacuum UV photoabsorption spectrum of 1,1-C2H2F2 has been examined in detail between 6eV and 25eV photon energy by using synchrotron radiation. The broad band observed at 7.52eV includes the π→π∗ and the 2b1→3s Rydberg transitions. An analysis is proposed and applied to the fine structure belonging to these transitions. For the π(2b1)→π∗ transition, one long vibrational progression is observed with ω2=1475±80cm−1 combined with one quantum of vibration of ω4=976±24cm−1 and its adiabatic excitation energy is determined at 6.742eV. The 2b1→3s Rydberg transition is characterized by a single progression with ω2=1532±80cm−1 starting at 6.957eV. These vibrations are ascribed to the CC and C–F stretching motions respectively. The abundant structure observed between 8.2eV and 11.2eV has been analyzed in terms of vibronic transitions to ns (δ=0.98) and two different np (δ=0.50 and 0.31) and nd (δ=0.13 and 0.044) Rydberg series all converging to the 1,1-C2H2F2+(X˜2B1) ionic ground state. An analysis of the associated vibrational structure of the individual Rydberg states has been attempted providing average values of the wavenumbers ω2=1549±16cm−1, ω4=839±40cm−1 and ω5=589±16cm−1. Eight other Rydberg states were also analyzed. The vacuum UV spectrum of 1,1-C2H2F2 has been recorded for the first time above 11.2eV and up to 25eV. Several broad and strong bands are tentatively assigned to transitions to Rydberg states which are members of Rydberg series converging to excited ionic states of 1,1-C2H2F2. For several of these Rydberg states vibrational, structures are observed and assignments are proposed.

Medium and high resolution vacuum UV photoabsorption spectroscopy of methyl iodide (CH 3I) and its deuterated isotopomers CD 3I and CH 2DI. A Rydberg series analysis

The vacuum UV photoabsorption spectrum of CH 3I has been investigated between 5 eV and 20 eV. Numerous vibronic transitions are observed. In the high 10–20 eV photon energy range weak to very weak diffuse bands are observed and ascribed to electronic transitions from 3a 1, 1e and 2a 1 to Rydberg orbitals. In the 6–10.5 eV photon energy range more than 200 sharp and strong to weak lines have been observed. Several photon energy ranges were explored under high resolution conditions allowing us to observe many series up to high values of the principal quantum number n. They are assigned to vibrationless Rydberg transitions and classified into two groups converging to the two components of the spin–orbit split X ˜ 2 E state of CH 3I +. These two groups consist of six different Rydberg series, i.e., nsa 1, npa 1, npe, nda 1, nde and nf. A very close correlation has been established between the term values of the Rydberg states in CH 3I and in Xe for ns, np, nd and nf Rydberg transitions. For the first time, the same measurements have been performed and the interpretation has been proposed for the photoabsorption spectrum of CH 2DI and CD 3I in the 6–10.5 eV photon energy range. For these two species, ionization energies are deduced: for the X ˜ 2 E 3/2 and X ˜ 2 E 1/2 states of CH 2DI, these are IE ad = 9.544 eV and IE ad = 10.168 eV, respectively, and the corresponding energies are 9.552 eV and 10.173 eV in CD 3I.

The vacuum UV photoabsorption spectroscopy of vinyl fluoride (C 2H 3F): The vibrational fine structure and its analysis

The vacuum UV photoabsorption spectrum of C 2H 3F has been examined in detail between 6 eV and 25 eV photon energy by using synchrotron radiation. The analysis of the data is supported by ab initio quantum mechanical calculations applied to valence and Rydberg excited states of C 2H 3F. At 7.6 eV the π → π ∗ and the 2a″ → 3s transitions are observed. An analysis is proposed and applied to the mixed fine structure belonging to these transitions. For the π → π ∗ transition one single long vibrational progression is observed with hcω e = 95 ± 7 meV (766 ± 56 cm −1) and its adiabatic excitation energy is 6.892 eV (55 588 cm −1). The 2a″ → 3s transition is characterized by a single short progression with hcω e = 167 ± 10 meV (1350 ± 80 cm −1) starting at 6.974 eV (56 249 cm −1). From the present ab initio calculations these two wavenumbers best correspond to the vibrational modes v 9 (CH 2 rock in-plane, FCC-bend) and v 6 (CH 2 rock in-plane, CF stretch) calculated at 615 cm −1 in the π ∗ state and 1315 cm −1 in the ( 2A″)3s Rydberg state respectively. The C C stretching could not be excluded. The dense structured spectrum observed between 8.0 eV and 10.5 eV has been analyzed in terms of vibronic transitions to Rydberg states all converging to the C 2 H 3 F + ( X ∼ A ″ ) ionic ground state. An analysis of the associated complex fine structure of the individual Rydberg states has been attempted providing average values of the wavenumbers, e.g., for the ( 2A″)3p Rydberg state hcω 9 = 60 ± 1 meV (or 484 ± 8 cm −1), hcω 7 = 151 ± 7 meV (or 1218 ± 60 cm −1), hcω 4 = 191 ± 3 meV (or 1540 ± 24 cm −1). The assignment of hcω = 105 ± 5 meV (or 823 ± 40 cm −1) is discussed. These experimental values are in good agreement with the theoretical predictions for C 2H 3F + [R. Locht, B. Leyh, D. Dehareng, K. Hottmann, H. Baumgärtel, Chem. Phys. (in press)]. Above 10.5 eV and up to 25 eV several broad and strong bands are tentatively assigned to transitions to valence (V–V) and/or Rydberg (V–R) states converging to excited ionic states of C 2H 3F.

Assignment of the electronic spectrum of allene by a theoretical configuration interaction approach

Early assignments of the vacuum UV photoabsorption spectrum of allene (1951–1972) gave a number of Rydberg series, but these series had limiting values for the ionization potential, which are incompatible (by up to 0.4 eV) with adiabatic IPs from molecular beam and photoelectron spectra (UV-PES). Following our recent re-analysis of the VUV spectrum of allene by systematic overlay of the VUV and UV-PES spectra, a number of new experimental assignments were obtained; specifically the following assignments provide a target for this theoretical study: 3s, 6.180; 3p, 6.960; 3d, 7.910; 4s, 8.076 eV. We now use multi-root configuration interaction calculations to determine the theoretical envelopes of both valence and Rydberg states. The CI calculations give a consistent picture of the Rydberg assignments as well as giving a clear indication of the positions of valence states. The second moment expectation values and oscillator strengths are used to determine the nature of the electronic states observed experimentally.

Assignment of the VUV electronic spectrum of allene

Early assignments of the vacuum UV photoabsorption spectrum of allene (1951–1972) gave a number of Rydberg series, but these series had limiting values for the ionization potential, which are incompatible (by up to 0.4 eV) with more recent adiabatic IPs from molecular beam and high resolution photoelectron spectra (UV-PES). We have performed a re-analysis of the VUV spectrum of allene by a combination of overlay of the VUV and UV-PES spectra which leads to new experimental assignments. Major differences in assignment to the early studies are reported, and these include the lowlying Rydberg states, which are now assigned as follows: 6.55 (3s), 7.2 (3p), 8.15 (3d), and 8.25 eV (4s), respectively, with further assignments up to 6s, 6p, and 6d.

The vacuum UV photoabsorption spectrum of methyl bromide (CH 3Br) and its perdeuterated isotopomer CD 3Br: a vibrational analysis

The fine structure of the vacuum UV photoabsorption spectrum of CH 3Br and CD 3Br has been analyzed in the 6.9–10.2 eV photon energy range. A large number of lines have been observed, classified and assigned to the vibrational excitation accompanying a series of Rydberg transitions. The effects of the Jahn–Teller distortion and of the spin–orbit splitting of the ground electronic state of the ion have been considered. The former effect has been evaluated by ab initio calculations, showing that the 2E state (in the C 3v symmetry group) splits into 2A′ and 2A″ states in the C S symmetry group. Even though the energy difference of about 1 meV is extremely small, the 2A′ state is energetically the lowest component whereas the 2A″ is found to be a transition state. The Jahn–Teller stabilization energy and the wavenumbers associated with all vibrational modes have been calculated. Experimentally, the entire fine structure could be described in terms of three vibrational modes, i.e., hcω 4 = 146 ± 6 meV (1178 ± 48 cm −1), hcω 5 = 107 ± 6 meV (863 ± 48 cm −1) and hcω 6 = 71 ± 4 meV (572 ± 32 cm −1), respectively, as resulting from an average over all analyzed Rydberg states. In CD 3Br the corresponding energies are hcω 4 = 104 ± 3 meV (839 ± 24 cm −1), hcω 5=82 ± 4 meV (661 ± 32 cm −1) and hcω 6 = 62 ± 4 meV (500 ± 32 cm −1). These values are in fairly good agreement with those predicted by the present ab initio calculations for the ionic ground state. The experimental isotopic ratio ρ i = [ ω/ ω isot] i is 1.15 ± 0.14, 1.31 ± 0.14 and 1.39 ± 1.0 and is predicted to be 1.04, 1.34 and 1.36, respectively, for ω 6, ω 5 and ω 4. On the basis of the present study an alternative assignment of the CH 3 Br + ( X ˜ 2 E ) photoelectron band structure is proposed.

The vacuum UV photoabsorption spectrum of methyl bromide (CH 3Br) and its perdeuterated isotopomer CD 3Br: a Rydberg series analysis

The vacuum UV photoabsorption spectrum of CH 3Br has been recorded between 6 and 25 eV. A large number of vibronic bands are observed. They were partly ascribed to vibrationless Rydberg transitions. In the high photon energy range of 12–25 eV, very weak diffuse bands are mostly assigned to transitions from the 3a 1, 1e and (2a 1 + 1a 1) to 3s orbitals. In the 6–12 eV photon energy range, numerous weak to strong bands are observed. The sharpness is very variable over the entire spectral region. In a first step, the interpretation of the spectrum and the assignment of the Rydberg transitions is based on the simple Rydberg formula. The observed features are classified in two groups of four series, each converging to one of the two spin–orbit components of the X ˜ 2 E state of CH 3Br +. Rydberg series of nsa 1, npa 1, npe, nd and possibly nf characters are observed. The same measurements have been made for the first time on CD 3Br in the 6–12 eV photon energy range. The observed features are classified into the same Rydberg series characterized by nearly the same δ values. Ionization energies for CD 3Br X ˜ 2 E 3 / 2 at 10.565 eV and X ˜ 2 E 1 / 2 at 10.902 eV are deduced. In a second step, we fitted the experimental data to an energy expression taking into account both the exchange interaction and the spin–orbit coupling. Already states with n = 6 are found to correspond to Hund’s case (c). Constant values of the quantum defects are deduced.

The vacuum UV photoabsorption spectrum of methyl chloride (CH 3Cl) and its perdeuterated isotopomer CD 3Cl I. A Rydberg series analysis

The vacuum UV photoabsorption spectrum of CH 3Cl has been recorded between 6 and 25 eV. A large number of vibronic bands are observed. These were partly ascribed to vibrationless Rydberg transitions. In the high photon energy range of 12–25 eV, very weak diffuse bands are mostly assigned to transitions from the 3a 1, 1e and 2a 1 to 3s orbitals. In the 6–12 eV photon energy range, numerous weak to strong bands are observed. The sharpness is very variable over the entire spectral region. In a first step, the interpretation of the spectrum and the assignment of the Rydberg transitions is based on the simple Rydberg formula. The observed features are classified in two groups of four series, each converging to one of the two spin–orbit components of the X 2 E state of CH 3Cl +. Rydberg series of nsa 1 ( δ=1.069 and 1.064), npa 1 ( δ=0.68 and 0.66), npe ( δ=0.438 and 0.427) and nd ( δ=−0.040 and −0.092) characters are observed. The same measurements have been made for the first time on CD 3Cl in the 6–12 eV photon energy range. The same Rydberg transitions are observed. Analogous series are characterized by about the same δ values: nsa 1 ( δ=1.038 and 0.968), npa 1 ( δ=0.65 and 0.61), npe ( δ=0.458 and 0.462) and nd ( δ=−0.004 and −0.082). Ionization energies for CD 3Cl X 2 E 3/2 at 11.320 eV and X 2 E 1/2 at 11.346 eV are deduced. In a second step, we fitted the experimental data for the nsa 1 and npa 1 states to an energy expression taking into account both the exchange interaction and the spin–orbit coupling. This accounts for the progressive switching from Hund's case (a) to Hund's case (c).

The vacuum UV photoabsorption spectrum of methyl chloride (CH 3Cl) and its perdeuterated isotopomer CD 3Cl : II. A vibrational analysis

The fine structure of the vacuum UV photoabsorption spectrum of CH 3Cl and CD 3Cl has been analyzed in the 7.5–10.5 eV photon energy range. A large number of lines have been observed, classified and assigned to the vibrational excitation accompanying a series of Rydberg transitions. The vibronic transitions involve both Jahn–Teller distortion and spin–orbit splitting. The former effect has been evaluated by ab initio calculations showing that the 2 E state (in the C 3v symmetry group) splits into 2 A ′ and 2 A ′′ states in the C S symmetry group. The 2 A ′ state is energetically the lowest component whereas the 2 A ′′ is found to be a transition state. The Jahn–Teller stabilization energy and the wave numbers associated with all vibrational modes have been calculated. Experimentally, the entire fine structure could be described by using three vibrational modes, i.e. hcω 5=104±7 meV, hcω 6=77±7 meV and hcω=162±3 meV respectively, as resulting from an average over all the analyzed Rydberg states. In CD 3Cl the corresponding energies are hcω 5=81±4 meV, hcω 6=66±5 meV and hcω=124±4 meV. These values are in good agreement with those predicted by the present ab initio calculations for the the ion ground state. However, the agreement is not so good for the wave number ω which could be assigned to the ν 3 or to the ν 4 vibrational modes. Though some arguments favour ν 4 (the CH 3 umbrella mode), within the error limits on the present measurements it is formally not possible to ascribe this wave number to one of these two vibrations.

Vacuum-UV absorption spectrum of a laser-produced chromiumplasma: 3p-subshell photoabsorption by Cr2+ ions

The dual laser plasma photoabsorption technique has been used tomeasure the time-resolved vacuum-UV photoabsorption spectrum ofa chromium plasma. Resonant photoabsorption cross sections, constructedwith the aid of Hartree-Fock calculations, and weighted in accordancewith the plasma temperature, have been used to produce the syntheticCr2+ spectra. The relevant plasma temperature and ionization balanceare obtained from simple analytical models for various times during theexpansion phase of the plasma plume. The experimental spectra taken atdelays of 32, 62 and 90 ns compare well with Cr2+spectra computedfor corresponding predicted temperatures. It is found that in order toproduce synthetic spectra that match experiment well, it is necessary totake into account absorption from many states belonging to the Cr2+ground state configuration 3p63d4, while states from the nearestmetastable configuration 3p63p34s make a negligible contribution.

About the vacuum UV photoabsorption spectrum of methyl fluoride (CH 3F): the fine structure and its vibrational analysis

The vacuum UV photoabsorption spectrum of CH 3F has been recorded between 7 and 24 eV and has been analyzed in detail. Broad and structureless peaks are observed over the entire photon energy range. They are all assigned to transitions to Rydberg states, members of series converging successively to the 2e −1, 5a −1 and 1e −1 ionization limits. These features are underlying very long series of narrow and weak structures in the range of 10–13.2 eV. Through a continuum subtraction procedure, about 70 lines could clearly be identified. These have been assigned to long vibrational progressions belonging to Rydberg states corresponding to the 2e→3p and 2e→6s/5d configurations. These states are split by a Jahn–Teller distortion. Ab initio calculations lead to a Jahn–Teller stabilization energy of about 0.84 eV. The main features observed in the two bands are assigned to one single long vibrational progression of hcω≃0.160 eV (1 290 cm −1) related to the C–F Jahn–Teller inactive stretching vibration. The remaining structure is assigned to the low excitation of the Jahn–Teller active vibrational modes, i.e. the bending modes ν 5 and ν 6 characterized by hcω 5≃0.120 eV (970 cm −1) and hcω 6≃0.082 eV (660 cm −1). Leaning on the present results and assignments previously reported photoelectron spectroscopic data on CH 3F + [L. Karlsson, R. Jadrny, L. Mattsson, F.T. Chau, K. Siegbahn, Phys. Scripta 16 (1977) 225] have been reconsidered and reinterpreted.