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Three spin-orbit states, 1²Π1/2, 2²Π3/2, and 2²Π1/2, of chloroiodine anion (ICl-) formed by lowenergy electron attachment in the Franck-Condon region are associated with the dissociative limits of I- (¹S0) and Cl (²P3/2) or Cl* (²P1/2) fragments. Within the adiabatic scheme, the presumptive Π-symmetry of the fragment angular distributions is dramatically changed to be the Π-Σ mixing symmetry, due to the significant spin-orbit interaction effect on the electronic state couplings of ICl-. The present experimental approach also enables us to separate the contributions of different electronic states from the mixed states, providing a crucial method for quantitatively evaluating the configuration-interaction wavefunctions. [ABSTRACT FROM AUTHOR]

The vacuum ultraviolet (VUV) laser pulsed field ionization-photoelectron (PFI-PE) spectrum of trans-1,3-butadiene (trans-CH2==CHCH==CH2) has been measured in the region of 0–1700 cm-1 above its ionization energy (IE) to probe the vibrational modes νi+ (i=1–18) of trans-CH2==CHCH==CH2+. The high-frequency vibrational modes νi+ (i=19, 22, and 23) of trans-CH2==CHCH==CH2+ have also been probed by the VUV-infrared-photoinduced Rydberg ionization (VUV-IR-PIRI) measurement. On the basis of the semiempirical simulation of the origin VUV-PFI-PE band, the IE(trans-CH2==CHCH==CH2) is determined to be 73 150.1±1.5 cm-1 (9.06946±0.00019 eV). This value has been used to benchmark the state-of-the-art theoretical IE prediction based on the CCSD(T,Full)/CBS procedures, the calculation of which is reported in the present study. The vibrational bands observed in the VUV-PFI-PE and VUV-IR-PIRI spectra were assigned based on ab initio anharmonic vibrational frequencies and Franck–Condon factor calculations for the photoionization transitions. Combining the VUV-PFI-PE and VUV-IR-PIRI measurements, 17 fundamental vibrational frequencies of trans-CH2==CHCH==CH2+ have been determined, including ν1+=182±3, ν2+=300±3, ν3+=428±3, ν4+=514±3, ν5+=554±5, ν6+=901±3, ν7+=928±5, ν8+=994±3, ν9+=1008±5, ν10+=1094±5, ν13+=1258±3, ν14+=1293±3, ν16+=1479±3, ν18+=1620±3, ν19+=2985±10, ν22+=3030±10, and ν23+=3105±10 cm-1. [ABSTRACT FROM AUTHOR]

The pulsed field ionization-photoelectron (PFI-PE) spectrum of bromochloromethane (CH2BrCl) in the region of 85 320–88 200 cm-1 has been measured using vacuum ultraviolet laser. The vibrational structure resolved in the PFI-PE spectrum was assigned based on ab initio quantum chemical calculations and Franck-Condon factor predictions. At energies 0–1400 cm-1 above the adiabatic ionization energy (IE) of CH2BrCl, the Br–C–Cl bending vibration progression (ν1+=0–8) of CH2BrCl+ is well resolved and constitutes the major structure in the PFI-PE spectrum, whereas the spectrum at energies 1400–2600 cm-1 above the IE(CH2BrCl) is found to exhibit complex vibrational features, suggesting perturbation by the low lying excited CH2BrCl+(Ã 2A″) state. The assignment of the PFI-PE vibrational bands gives the IE(CH2BrCl)=85 612.4±2.0 cm-1 (10.6146±0.0003 eV) and the bending frequencies ν1+(a1′)=209.7±2.0 cm-1 for CH2BrCl+(X 2A′). We have also examined the dissociative photoionization process, CH2BrCl+hν→CH2Cl++Br+e-, in the energy range of 11.36–11.57 eV using the synchrotron based PFI-PE-photoion coincidence method, yielding the 0 K threshold or appearance energy AE(CH2Cl+)=11.509±0.002 eV. Combining the 0 K AE(CH2Cl+) and IE(CH2BrCl) values obtained in this study, together with the known IE(CH2Cl), we have determined the 0 K bond dissociation energies (D0) for CH2Cl+–Br (0.894±0.002 eV) and CH2Cl–Br (2.76±0.01 eV). We have also performed CCSD(T, full)/complete basis set (CBS) calculations with high-level corrections for the predictions of the IE(CH2BrCl), AE(CH2Cl+), IE(CH2Cl), D0(CH2Cl+–Br), and D0(CH2Cl–Br). The comparison between the theoretical predictions and experimental determinations indicates that the CCSD(T, full)/CBS calculations with high-level corrections are highly reliable with estimated error limits of <17 meV. [ABSTRACT FROM AUTHOR]

The vacuum ultraviolet (VUV) laser pulsed field ionization photoelectron (PFI-PE) spectrum of cis-dichloroethene (cis-ClCH==CHCl) has been measured in the energy region of 77 600–79 500 cm-1. On the basis of the semiempirical simulation of the origin PFI-PE band, we have obtained the IE(cis-ClCH==CHCl) to be 77 899.5±2.0 cm-1 (9.658 39±0.000 25 eV). The assignment of the vibrational bands resolved in the VUV-PFI-PE spectrum are guided by high-level ab initio calculations of the vibrational frequencies for cis-ClCH==CHCl+ and the Franck-Condon factors for the ionization transitions. Combining the results of the present VUV-PFI-PE measurement and the recent VUV-infrared-photoinduced Rydberg ionization study [P. Wang et al., J. Chem. Phys. 124, 064310 (2006)], the vibrational frequencies for eleven of the twelve vibrational modes of cis-ClCH==CHCl+ have been experimentally determined: ν1+(a1)=181 cm-1, ν2+(a2)=277 cm-1, ν3+(b2)=580 cm-1, ν4+(b1)=730 cm-1, ν5+(a1)=810 cm-1, ν6+(a2)=901 cm-1, ν8+(a1)=1196 cm-1, ν9+(b2)=1348 cm-1, ν10+(a1)=1429 cm-1, ν11+(b2)=3067 cm-1, and ν12+(a1)=3090 cm-1). These values are compared to theoretical anharmonic vibrational frequencies obtained at the MP2/6-311G(2df,p) and CCSD(T)/6-311G(2df,p) levels. The IE prediction for cis-ClCH==CHCl has also been calculated with the wave function based CCSD(T)/CBS method, which involves the approximation to the complete basis set (CBS) and the high-level correlation corrections. The theoretical IE(cis-ClCH==CHCl)=9.668 eV thus obtained is found to have a deviation of less than 10 meV with respect to the experimental IE value. [ABSTRACT FROM AUTHOR]

We have developed an effusive laser photodissociation radical source, aiming for the production of vibrationally relaxed radicals. Employing this radical source, we have measured the vacuum ultraviolet (VUV) photoionization efficiency (PIE) spectrum of the propargyl radical (C3H3) formed by the 193 nm excimer laser photodissociation of propargyl chloride in the energy range of 8.5-9.9 eV using high-resolution (energy bandwidth=1 meV) multibunch synchrotron radiation. The VUV-PIE spectrum of C3H3 thus obtained is found to exhibit pronounced autoionization features, which are tentatively assigned as members of two vibrational progressions of C3H3 in excited autoionizing Rydberg states. The ionization energy (IE=8.674±0.001 eV) of C3H3 determined by a small steplike feature resolved at the photoionization onset of the VUV-PIE spectrum is in excellent agreement with the IE value reported in a previous pulsed field ionization-photoelectron study. We have also calculated the Franck-Condon factors (FCFs) for the photoionization transitions C3H3+(...;vi, 1 - 12) ← C3H3(...). The comparison between the pattern of FCFs and the autoionization peaks resolved in the VUV-PIE spectrum of C3H3 points to the conclusion that the resonance-enhanced autoionization mechanism is most likely responsible for the observation of pronounced autoionization features. We also present here the VUV-PIE spectra for the mass 39 ions observed in the VUV synchrotron-based photoionization mass spectrometric sampling of several premixed flames. The excellent agreement of the IE value and the pattern of autoionizing features of the VUV-PIE spectra observed in the photodissociation and flames studies has provided an unambiguous identification of the propargyl radical as an important intermediate in the premixed combustion flames. The discrepancy found between the PIE spectra obtained in flames and photodissociation at energies above the IE(C3H3) suggests that the PIE spectra obtained in flames might have contributions from the photoionization of vibrationally excited C3H3 and/or the dissociative photoionization processes involving larger hydrocarbon species formed in flames. [ABSTRACT FROM AUTHOR]

We have measured the infrared (IR) vibrational spectrum for cis-dichloroethene (cis-ClCH==CHCl) in excited Rydberg states with the effective principal quantum numbers n*=9, 13, 17, 21, 28, and 55 using the vacuum ultraviolet-IR-photoinduced Rydberg ionization (VUV-IR-PIRI) scheme. Although the IR frequencies observed for the vibrational bands ν11* (asymmetric C–H stretch) and ν12* (symmetric C–H stretch) are essentially unchanged for different n* states, suggesting that the IR absorption predominantly involves the ion core and that the Rydberg electron behaves as a spectator; the intensity ratio for the ν11* and ν12* bands [R(ν11*/ν12*)] is found to decrease smoothly as n* is increased. This trend is consistent with the results of a model ab initio quantum calculation of R(ν11*/ν12*) for excited cis-ClCH==CHCl in n*=3–18 states and the MP2/6-311++G(2df,p) calculations of R(ν11/ν12) and R(ν11+/ν12+), where R(ν11/ν12)[R(ν11+/ν12+)] represents the intensity ratio of the ν11(ν11+) asymmetric C–H stretching to the ν12(ν12+) symmetric C–H stretching vibrational bands for cis-ClCH==CHCl (cis-ClCH==CHCl+). We have also measured the IR-VUV-photoion (IR-VUV-PI) and IR-VUV-pulsed field ionization-photoelectron depletion (IR-VUV-PFI-PED) spectra for cis-ClCH==CHCl. These spectra are consistent with ab initio calculations, indicating that the IR absorption cross section for the ν12 band is negligibly small compared to that for the ν11 band. While the VUV-IR-PIRI measurements have allowed the determination of ν11+=3067±2 cm-1, ν12+=3090±2 cm-1, and R(ν11+/ν12+)≈1.3 for cis-ClCH==CHCl+, the IR-VUV-PI and IR-VUV-PFI-PED measurements have provided the value ν11=3088.5±0.2 cm-1 for cis-ClCH==CHCl. [ABSTRACT FROM AUTHOR]

The ionization energies (IEs) for the 2-propyl (2-C3H7), phenyl (C6H5), and benzyl (C6H5CH2) radicals have been calculated by the wave-function-based ab initio CCSD(T)/CBS approach, which involves the approximation to the complete basis set (CBS) limit at the coupled cluster level with single and double excitations plus quasiperturbative triple excitation [CCSD(T)]. The zero-point vibrational energy correction, the core-valence electronic correction, and the scalar relativistic effect correction have been also made in these calculations. Although a precise IE value for the 2-C3H7 radical has not been directly determined before due to the poor Franck-Condon factor for the photoionization transition at the ionization threshold, the experimental value deduced indirectly using other known energetic data is found to be in good accord with the present CCSD(T)/CBS prediction. The comparison between the predicted value through the focal-point analysis and the highly precise experimental value for the IE(C6H5CH2) determined in the previous pulsed field ionization photoelectron (PFI-PE) study shows that the CCSD(T)/CBS method is capable of providing an accurate IE prediction for C6H5CH2, achieving an error limit of 35 meV. The benchmarking of the CCSD(T)/CBS IE(C6H5CH2) prediction suggests that the CCSD(T)/CBS IE(C6H5) prediction obtained here has a similar accuracy of 35 meV. Taking into account this error limit for the CCSD(T)/CBS prediction and the experimental uncertainty, the CCSD(T)/CBS IE(C6H5) value is also consistent with the IE(C6H5) reported in the previous HeI photoelectron measurement. Furthermore, the present study provides support for the conclusion that the CCSD(T)/CBS approach with high-level energy corrections can be used to provide reliable IE predictions for C3–C7 hydrocarbon radicals with an uncertainty of ±35 meV. Employing the atomization scheme, we have also computed the 0 K (298 K) heats of formation in kJ/mol at the CCSD(T)/CBS level for 2-C3H7/2-C3H7+ ,C6H5/C6H5+, and C6H5CH2/C6H5CH2+ to be 105.2/822.7 (90.0/806.4), 351.4/1148.5 (340.4/1138.8), and 226.2/929.0 (210.3/912.7), respectively. Comparing these values with the available experimental values, we find that the discrepancies for the 0 and 298 K heats of formation values are ≤2.6 kJ/mol for 2-C3H7/2-C3H7+,≤4.1 kJ/mol for C6H5/C6H5+, and ≤3.2 kJ/mol for C6H5CH2/C6H5CH2+. [ABSTRACT FROM AUTHOR]

This work determines the dissociation barrier height for CH2CHCO→CH2CH+CO using two-dimensional product velocity map imaging. The CH2CHCO radical is prepared under collision-free conditions from C–Cl bond fission in the photodissociation of acryloyl chloride at 235 nm. The nascent CH2CHCO radicals that do not dissociate to CH2CH+CO, about 73% of all the radicals produced, are detected using 157-nm photoionization. The Cl(2P3/2) and Cl(2P1/2) atomic fragments, momentum matched to both the stable and unstable radicals, are detected state selectively by resonance-enhanced multiphoton ionization at 235 nm. By comparing the total translational energy release distribution P(ET) derived from the measured recoil velocities of the Cl atoms with that derived from the momentum-matched radical cophotofragments which do not dissociate, the energy threshold at which the CH2CHCO radicals begin to dissociate is determined. Based on this energy threshold and conservation of energy, and using calculated C–Cl bond energies for the precursor to produce CH2CHCO or CH2CHCO, respectively, we have determined the forward dissociation barriers for the radical to dissociate to vinyl+CO. The experimentally determined barrier for CH2CHCO→CH2CH+CO is 21±2 kcal mol-1, and the computed energy difference between the CH2CHCO and the CH2CHCO forms of the radical gives the corresponding barrier for CH2CHCO→CH2CH+CO to be 23±2 kcal mol-1. This experimental determination is compared with predictions from electronic structure methods, including coupled-cluster, density-functional, and composite Gaussian-3-based methods. The comparison shows that density-functional theory predicts too low an energy for the CH2CHCO radical, and thus too high a barrier energy, whereas both the Gaussian-3 and the coupled-cluster methods yield predictions in good agreement with experiment. The experiment also shows that acryloyl chloride can be used as a photolytic precursor at 235 nm of thermodynamically stable CH2CHCO radicals, most with an internal energy distribution ranging from ≈3 to ≈21 kcal mol-1. We discuss the results with respect to the prior work on the O(3P)+propargyl reaction and the analogous O(3P)+allyl system. [ABSTRACT FROM AUTHOR]

The ionization energies for methylene (CH2), methyl (CH3), ethynyl (C2H), vinyl (C2H3), ethyl (C2H5), propargyl (C3H3), and allyl (C3H5) radicals have been calculated by the wave-function-based ab initio CCSD(T)/CBS approach, which involves the approximation to the complete basis set (CBS) limit at the coupled-cluster level with single and double excitations plus a quasiperturbative triple excitation [CCSD(T)]. When it is appropriate, the zero-point vibrational energy correction, the core–valence electronic correction, the scalar relativistic effect correction, the diagonal Born–Oppenheimer correction, and the high-order correlation correction have also been made in these calculations. The comparison between the computed ionization energy (IE) values and the highly precise experimental IE values determined in previous pulsed field ionization-photoelectron (PFI-PE) studies indicates that the CCSD(T)/CBS method is capable of providing accurate IE predictions for these hydrocarbon radicals achieving error limits well within ±10 meV. The benchmarking of the CCSD(T)/CBS IE predictions by the PFI-PE experimental results also lends strong support for the conclusion that the CCSD(T)/CBS approach with high-level energy corrections can serve as a valuable alternative for reliable IE determination of radicals, particularly for those radicals with very unfavorable Franck–Condon factors for photoionization transitions near their ionization thresholds. [ABSTRACT FROM AUTHOR]

The ionization energies for methylene (CH2), methyl (CH3), ethynyl (C2H), vinyl (C2H3), ethyl (C2H5), propargyl (C3H3), and allyl (C3H5) radicals have been calculated by the wave-function-based ab initio CCSD(T)/CBS approach, which involves the approximation to the complete basis set (CBS) limit at the coupled-cluster level with single and double excitations plus a quasiperturbative triple excitation [CCSD(T)]. When it is appropriate, the zero-point vibrational energy correction, the core–valence electronic correction, the scalar relativistic effect correction, the diagonal Born–Oppenheimer correction, and the high-order correlation correction have also been made in these calculations. The comparison between the computed ionization energy (IE) values and the highly precise experimental IE values determined in previous pulsed field ionization-photoelectron (PFI-PE) studies indicates that the CCSD(T)/CBS method is capable of providing accurate IE predictions for these hydrocarbon radicals achieving error limits well within ±10 meV. The benchmarking of the CCSD(T)/CBS IE predictions by the PFI-PE experimental results also lends strong support for the conclusion that the CCSD(T)/CBS approach with high-level energy corrections can serve as a valuable alternative for reliable IE determination of radicals, particularly for those radicals with very unfavorable Franck–Condon factors for photoionization transitions near their ionization thresholds. [ABSTRACT FROM AUTHOR]

The preparation of methyl iodide (CH3I) in selected rovibrational states [ν7=1 (C-H stretch); J] by infrared (IR) excitation prior to vacuum ultraviolet (VUV) photoionization has greatly simplified the observed pulsed field ionization-photoelectron (PFI-PE) spectra, allowing the direct determination of the rotational constants B+(C+)=0.254±0.003 cm-1 for CH3I+(X 2E3/2;ν7+) and the ionization energy (76 896.9±0.2 cm-1) for CH3I+(X 2E3/2;ν7+=1,J+=3/2)←CH3I(X 1A1;ν7=1,J=0). The IR-VUV-PFI-PE and IR-VUV-photoion measurements also provide relative state-to-state (ν7+=1, J+←ν7=1, J) cross sections for the photoionization process. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

By employing the high-resolution pulsed field ionization-photoelectron (PFI-PE)-photoion coincidence method, we have examined the unimolecular dissociation reaction of energy-selected C2H3Br+ to form C2H3+ + Br near its threshold. The analysis of the breakdown curves for C2H3Br+ and C2H3+ yields a value of 11.9010±0.0015 eV for the 0 K dissociative photoionization threshold or appearance energy (AE) for C2H3+ from C2H3Br. This AE (C2H3+) value, together with the ionization energy (IE) for C2H3Br 9.8200±0.0015 eV obtained by PFI-PE and threshold photoelectron (TPE) measurements, has allowed the determination of the 0 K dissociation energy (D0) for the C2H3+—Br bond to be 2.081±0.002 eV. The 0 K AE (C2H3+) from C2H3Br obtained in this study corresponds to ΔH°f0(C2H3+)=1123.7±1.9 kJ/mol. Combining the latter value and the known ΔH°f0(C2H3)=306.7±2.1 kJ/mol, we calculated a value of 8.468±0.029 eV for the IE(C2H3), which is in accord with the result obtained in the previous photoionization efficiency study. We have also carried out high-level ab initio calculations for the IE(C2H3) at the Gaussian-3 and the CCSD(T,full)/CBS level of theory. The CCSD(T,full)/CBS prediction of 8.487 eV for the IE(C2H3→bridged-C2H3+) is in good agreement with the IE(C2H3) value derived in the present experiment. Combining the 0 K AE(C2H3+)=11.9010±0.0015 eV and the IE(C2H3)=8.468±0.029 eV yields the value of 3.433±0.029 eV for D0(C2H3-Br). We have also recorded the TPE spectrum of C2H3Br in the energy range of 9.80–12.20 eV. Members (n=5–14) of four autoionizing Rydberg series converging to the C2H3Br+(Ã2A′) state are observed in the TPE spectrum. The analysis of the converging limit of these Rydberg series and the vibrational TPE bands for C2H3Br+(Ã2A′) has provided more precise values for the ν6+ (1217±10 cm-1) and ν8+(478±8 cm-1) modes and the IE (10.9156±0.0010 eV) for the formation of C2H3Br+(Ã2A′) from C2H3Br. [ABSTRACT FROM AUTHOR]

The dissociation of energy-selected ND3+ to form ND2++D near its threshold has been investigated using the pulsed field ionization-photoelectron (PFI-PE)-photoion coincidence method. The breakdown curves for ND3+ and ND2+ give a value of 15.891±0.001 eV for the 0 K dissociation threshold or appearance energy (AE) for ND2+ from ND3. We have also measured the PFI-PE vibrational bands for ND3+(X;v2+=0, 1, 2, and 3), revealing partially resolved rotational structures. The simulation of these bands yields precise ionization energies (IEs) for ND3+ X(0,v2+=0–3,0,0)←ND3 X(0,0,0,0). Using the 0 K AE (ND2+) and IE(ND3)=10.200±0.001 eV determined in the present study, together with the known 0 K bond dissociation energy for ND3 [D0(D–ND2)=4.7126±0.0025 eV], we have determined the D0(ND2+–D), IE(ND2), and 0 K heat of formation for ND2+ to be 5.691±0.001 eV, 11.1784±0.0025 eV, and 1261.82±0.4 kJ/mol, respectively. The PFI-PE spectrum is found to exhibit a steplike feature near the AE(ND2+), indicating that the dissociation of excited ND3+ at energies slightly above the dissociation threshold is prompt, occurring in the time scale ≤10-7 s, as observed for the NH3 system. The available energetic data for the NH2–NH2+ and NH3–NH3+ system are found to be in excellent accord with those for the ND2–ND2+ and ND3–ND3+ system after taking into account the zero-point vibrational energy corrections. This finding indicates that the thermochemical data for these two systems are reliable with well-founded error limits. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have demonstrated the two-color vacuum ultraviolet (VUV)-infrared (IR) photoinduced Rydberg ionization (PIRI) experiment. Trichloroethene (ClCH==CCl[sub 2]) and trans-2-butene (trans-CH[sub 3]CH==CHCH[sub 3]) were prepared in Rydberg states in the range of effective principal quantum number n[sup *]≈7–93 by VUV excitation prior to IR-induced autoionization. The observed VUV-IR-PIRI spectra are found to be independent of n[sup *], suggesting that the electron Rydberg orbital is conserved, i.e., the Rydberg electron is behaving as a spectator during the excitation process. The observed IR active C–H stretching vibrational frequencies ν[sub 12][sup +]=3072±5 cm[sup -1] for ClCH==CCl[sub 2][sup +] and ν[sub 23][sup +]=2908±3 cm[sup -1], ν[sub 25][sup +]=2990±10 cm[sup -1] and ν[sub 30][sup +]=3022±10 cm[sup -1] for trans-CH[sub 3]CH==CHCH[sub 3][sup +] are compared with predictions based on ab initio quantum-chemical procedures and density functional calculations. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

By employing the two-color infrared (IR)-vacuum ultraviolet (VUV) laser photoionization (PI) schemes, we have determined the C[Single_Bond]H stretching frequencies ν[sub 12]=3097.7 cm[sup -1] for trichloroethene (CHCl==CCl[sub 2]) and ν[sub 12][sup +]=3066 cm[sup -1] for CHCl==CCl[sub 2][sup +]. On the basis of the IR-VUV-pulsed field ionization-photoelectron (PFI-PE) measurement, the ratio of the state-to-state PI cross section for CHCl==CCl[sub 2][sup +](ν[sub 12][sup +]=1)←CHCl==CCl[sub 2](ν[sub 12]=1) to that for the formation of the ground state ion from the ground state neutral is determined to be 0.24. The VUV-PI efficiency (PIE) spectrum for CHCl==CCl[sub 2] and IR-VUV-PIE spectrum for CHCl==CCl[sub 2](ν[sub 12]=1) show that the state-to-state PI cross section for the formation of CHCl==CCl[sub 2][sup +] in the ground state from CHCl==CCl[sub 2](ν[sub 12]=1) is ≈0.006 that from CHCl==CCl[sub 2] in its ground state. As expected, the IR-VUV-PIE onset for CHCl==CCl[sub 2](ν[sub 12]=1) exhibits less hot-band tailing than the VUV-PIE onset for supersonically cooled CHCl==CCl[sub 2]. After taking into account the ν[sub 12] frequency, the ionization energy values for CHCl==CCl[sub 2] determined based on the IR-VUV-PIE and VUV-PFI-PE measurements are found to be in excellent agreement. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The single-photon pulsed field ionization photoelectron (PFI-PE) spectrum of trans-2-butene (trans-CH[sub 3]CH=CHCH[sub 3]) in the energy range of 73 500–75 850 cm-1 has been measured using vacuum ultraviolet laser sources. The semi-empirical simulation of fine structures resolved in the original PFI-PE band yields a value of 73 624.7±2.0 cm[sup -1] for the ionization energy (IE) of trans-2-butene. The vibrational bands for trans-CH[sub 3]CH=CHCH[sub 3][sup +] resolved in the PFI-PE spectrum are assigned based on ab initio calculations of the vibrational frequencies and Franck-Condon factors (FCFs) for ionization transitions. This assignment has provided reliable vibrational frequencies (ν[sub 1][sup +]=104 cm[sup -1], ν[sub 2][sup +]=127 cm[sup -1], ν[sub 3][sup +]=131 cm[sup -1], ν[sub 5][sup +]=484 cm[sup -1], ν[sub 8][sup +]=798 cm[sup -1], ν[sub 13][sup +]=1164 cm[sup -1], ν[sub 14][sup +]=1264 cm[sup -1], ν[sub 16][sup +]=1307 cm[sup -1], ν[sub 20][sup +]=1407 cm[sup -1], and ν[sub 22][sup +]=1567 cm[sup -1]) for trans-CH[sub 3]CH=CHCH[sub 3][sup +]. The PFI-PE spectrum is compared to the recently reported PFI-photoion (PFI-PI) spectrum for trans-2-butene. The major difference observed between the PFI-PE and PFI-PI spectra is that the intensities for excited vibrational bands were significantly suppressed or indiscernible in the PFI-PI spectrum, suggesting that the lifetimes for high-n Rydberg states associated with these excited vibrational bands were greatly reduced under the conditions used in the PFI-PI study. The experimental conditions used in the PFI-PI study also led to an IE value of about 20 cm-1 lower than that obtained in the PFI-PE measurement. We have also reassigned the vibrational bands resolved in the PFI-PE spectrum for cis-2-butene based on the FCF calculation and a more reliable set of theoretical vibrational frequencies. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The vacuum ultraviolet pulsed field ionization-photoelectron spectra of supersonically cooled cis-2-butene (cis-CH[sub 3]CH==CHCH[sub 3]) have been measured in the photon energy range of 73 560–75 460 cm-1. Using the ab initio theoretical rotational constants of cis-CH[sub 3]CH==CHCH[sub 3] and its cation (cis-CH[sub 3]CH==CHCH[sub 3][SUP ARRANGE="STAGGER"]+]) and a semiempirical simulation scheme, we have obtained a good fit of the origin vibrational band with partially resolved contours of rotational branches. After taking into account the Stark shift, the ionization energy of cis-CH[sub 3]CH==CHCH[sub 3] is determined to be 73 595.0±1.5 cm[sup -1]. Guided by ab initio vibrational frequency calculations, we have also assigned the vibrational bands observed for cis-CH[sub 3]CH==CHCH[sub 3][SUP ARRANGE="STAGGER"]+] in its ground state. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Using the high-resolution pulsed field ionization-photoelectron (PFI-PE) and PFI-PE-photoion coincidence (PFI-PEPICO) techniques, we have examined the formation of methyl cation (CH[sub 3][sup +]) from the dissociation of energy-selected CH[sub 3]X[sup +] (X=Br and I) near their dissociation thresholds. The breakdown diagrams for CH[sub 3]X thus obtained yield values of 12.834±0.002 eV and 12.269±0.003 eV for the 0 K dissociative threshold or appearance energy (AE) for CH[sub 3][sup +] from CH[sub 3]Br and CH[sub 3]I, respectively. Similar to the observation in PFI-PE studies of CH[sub 4], C[sub 2]H[sub 2], and NH[sub 3], the PFI-PE spectrum for CH[sub 3]Br exhibits a step at the 0 K AE for CH[sub 3][sup +], indicating that the dissociation of excited CH[sub 3]Br in high-n (>=100) Rydberg states at energies slightly above the dissociation threshold occurs in a time scale of ≤10[sup -7] s. The observed step is a confirmation of the 0 K AE(CH[sub 3][sup +]) from CH[sub 3]Br determined in the PFI-PEPICO study. The adiabatic ionization energies (IEs) for the CH[sub 3]Br[sup +](X˜ [sup 2]E[sub 3/2,1/2]) spin–orbit states were determined by PFI-PE measurements to be 10.5427±0.0010 and 10.8615±0.0010 eV, respectively, yielding the spin–orbit coupling constant to be 2571±4 cm-1. The AE(CH[sub 3][sup +]) values from CH[sub 3]Br and CH[sub 3]I and the IE[CH[sub 3]Br[sup +](X˜ [sup 2]E[sub 3/2])] value obtained here, when combined with the known IE of CH[sub 3] (9.8380±0.0004 eV) and IE[CH[sub 3]I[sup +](X˜ [sup 2]E[sub 3/2])] (9.5381±0.0001 eV), have allowed accurate determination of the 0 K bond dissociation energies for CH[sub 3]–Br (2.996±0.002 eV), CH[sub 3][sup +]–Br (2.291±0.002 eV), CH[sub 3]–I (2.431±0.003 eV), and CH[sub 3][sup +]–I (2.731±0.003 eV). Using the AE(CH[sub 3][sup +]) from CH[sub 3]Br and CH[sub 3]I, together with the known 0 K he... [ABSTRACT FROM AUTHOR]

Employing the newly developed high-resolution pulsed field ionization-photoelectron (PFI-PE)-photoion coincidence (PFI-PEPICO) technique, we have examined the dissociation of energy-selected NH[sub 3][sup +] to form NH[sub 2][sup +]+H near its threshold. The breakdown curves for NH[sub 2][sup +] and NH[sub 3][sup +] thus obtained yield a value of 15.765±0.001 eV for the 0 K dissociation threshold or appearance energy (AE) for NH[sub 2][sup +] from NH[sub 3]. This value, together with the known ionization energy (IE=10.1864±0.0001 eV) and 0 K bond dissociation energy (D[sub 0]=4.6017±0.0025 eV) for NH[sub 3], allows the determination of the D[sub 0](NH[sub 2][sup +]–H) and IE(NH[sub 2]), which are 5.5786±0.0010 and 11.1633±0.0025 eV, respectively. Using the known 0 K heats of formation (ΔH[sub f0][sup °]) for NH[sub 3] and H and the AE(NH[sub 2][sup +]), we obtain the ΔH[sub f0][sup °](NH[sub 2][sup +])=302.60±0.08 kcal/mol. The PFI-PE spectrum for NH[sub 3] exhibits a step-like feature at the 0 K AE(NH[sub 2][sup +]), indicating that the dissociation of excited NH[sub 3] in high-n (n>=100) Rydberg states at energies slightly above the dissociation threshold occurs on a time scale ≤10[sup -7] s. This step confirms the AE(NH[sub 2][sup +]) value derived from the PFI-PEPICO measurements. Highly accurate energetic data with well-founded error limits, such as those obtained in the present and other studies using the PFI techniques, are expected to play an important role for the development of the next generation of ab initio quantum computation procedures. This experiment has stimulated a state-of-the-art ab initio quantum chemical calculation [Dixon et al., J. Chem. Phys. 115, 2576 (2001)]. The comparison between theoretical predictions and the best experimental results for the NH[sub 2]/NH[sub 2][sup +] and NH[sub 3]/NH[sub 3][sup +] systems indicates that the accuracy of the computational scheme used ... [ABSTRACT FROM AUTHOR]

The pulsed field ionization-photoelectron (PFI-PE) spectrum of bromochloromethane (CH2BrCl) in the region of 85,320-88,200 cm-1 has been measured using vacuum ultraviolet laser. The vibrational structure resolved in the PFI-PE spectrum was assigned based on ab initio quantum chemical calculations and Franck-Condon factor predictions. At energies 0-1400 cm-1 above the adiabatic ionization energy (IE) of CH2BrCl, the Br-C-Cl bending vibration progression (nu1+=0-8) of CH2BrCl+ is well resolved and constitutes the major structure in the PFI-PE spectrum, whereas the spectrum at energies 1400-2600 cm-1 above the IE(CH2BrCl) is found to exhibit complex vibrational features, suggesting perturbation by the low lying excited CH2BrCl+(A 2A") state. The assignment of the PFI-PE vibrational bands gives the IE(CH2BrCl)=85,612.4+/-2.0 cm-1 (10.6146+/-0.0003 eV) and the bending frequencies nu1+(a1')=209.7+/-2.0 cm-1 for CH2BrCl+(X2A'). We have also examined the dissociative photoionization process, CH2BrCl+hnu-->CH2Cl++Br+e-, in the energy range of 11.36-11.57 eV using the synchrotron based PFI-PE-photoion coincidence method, yielding the 0 K threshold or appearance energy AE(CH2Cl+)=11.509+/-0.002 eV. Combining the 0 K AE(CH2Cl+) and IE(CH2BrCl) values obtained in this study, together with the known IE(CH2Cl), we have determined the 0 K bond dissociation energies (D0) for CH2Cl+-Br (0.894+/-0.002 eV) and CH2Cl-Br (2.76+/-0.01 eV). We have also performed CCSD(T, full)/complete basis set (CBS) calculations with high-level corrections for the predictions of the IE(CH2BrCl), AE(CH2Cl+), IE(CH2Cl), D0(CH2Cl+-Br), and D0(CH2Cl-Br). The comparison between the theoretical predictions and experimental determinations indicates that the CCSD(T, full)/CBS calculations with high-level corrections are highly reliable with estimated error limits of <17 meV.

This work determines the dissociation barrier height for CH2CHCO --> CH2CH + CO using two-dimensional product velocity map imaging. The CH2CHCO radical is prepared under collision-free conditions from C-Cl bond fission in the photodissociation of acryloyl chloride at 235 nm. The nascent CH2CHCO radicals that do not dissociate to CH2CH + CO, about 73% of all the radicals produced, are detected using 157-nm photoionization. The Cl(2P(3/2)) and Cl(2P(1/2)) atomic fragments, momentum matched to both the stable and unstable radicals, are detected state selectively by resonance-enhanced multiphoton ionization at 235 nm. By comparing the total translational energy release distribution P(E(T)) derived from the measured recoil velocities of the Cl atoms with that derived from the momentum-matched radical cophotofragments which do not dissociate, the energy threshold at which the CH2CHCO radicals begin to dissociate is determined. Based on this energy threshold and conservation of energy, and using calculated C-Cl bond energies for the precursor to produce CH2CHC*O or C*H2CHCO, respectively, we have determined the forward dissociation barriers for the radical to dissociate to vinyl + CO. The experimentally determined barrier for CH2CHC*O --> CH2CH + CO is 21+/-2 kcal mol(-1), and the computed energy difference between the CH2CHC*O and the C*H2CHCO forms of the radical gives the corresponding barrier for C*H2CHCO --> CH2CH + CO to be 23+/-2 kcal mol(-1). This experimental determination is compared with predictions from electronic structure methods, including coupled-cluster, density-functional, and composite Gaussian-3-based methods. The comparison shows that density-functional theory predicts too low an energy for the C*H2CHCO radical, and thus too high a barrier energy, whereas both the Gaussian-3 and the coupled-cluster methods yield predictions in good agreement with experiment. The experiment also shows that acryloyl chloride can be used as a photolytic precursor at 235 nm of thermodynamically stable CH2CHC*O radicals, most with an internal energy distribution ranging from approximately 3 to approximately 21 kcal mol(-1). We discuss the results with respect to the prior work on the O(3P) + propargyl reaction and the analogous O(3P) + allyl system.

The ionization energies for methylene (CH2), methyl (CH3), ethynyl (C2H), vinyl (C2H3), ethyl (C2H5), propargyl (C3H3), and allyl (C3H5) radicals have been calculated by the wave-function-based ab initio CCSD(T)/CBS approach, which involves the approximation to the complete basis set (CBS) limit at the coupled-cluster level with single and double excitations plus a quasiperturbative triple excitation [CCSD(T)]. When it is appropriate, the zero-point vibrational energy correction, the core-valence electronic correction, the scalar relativistic effect correction, the diagonal Born-Oppenheimer correction, and the high-order correlation correction have also been made in these calculations. The comparison between the computed ionization energy (IE) values and the highly precise experimental IE values determined in previous pulsed field ionization-photoelectron (PFI-PE) studies indicates that the CCSD(T)/CBS method is capable of providing accurate IE predictions for these hydrocarbon radicals achieving error limits well within +/-10 meV. The benchmarking of the CCSD(T)/CBS IE predictions by the PFI-PE experimental results also lends strong support for the conclusion that the CCSD(T)/CBS approach with high-level energy corrections can serve as a valuable alternative for reliable IE determination of radicals, particularly for those radicals with very unfavorable Franck-Condon factors for photoionization transitions near their ionization thresholds.

The vacuum-ultraviolet (VUV) pulsed-field ionization-photoelectron (VUV-PFI-PE) spectra of trans-1-bromopropene (trans-CH(3)CH[Double Bond]CHBr) and cis-1-bromopropene (cis-CH(3)CH[Double Bond]CHBr) have been measured in the energy region of 74 720-76 840 cm(-1). The simulation of fine structures observed in the origin VUV-PFI-PE vibrational bands of these molecules has provided the ionization energies (IEs) of trans-1-bromopropene and cis-1-bromopropene to be 74 779.3+/-2.0 cm(-1) (9.2715+/-0.0002 eV) and 75 140.2+/-2.0 cm(-1) (9.3162+/-0.0002 eV), respectively. The vibrational bands resolved in these VUV-PFI-PE spectra at energies 0-1700 cm(-1) above the IEs of trans-1-bromopropene and cis-1-bromopropene have been assigned based on theoretical vibrational frequencies and calculated Franck-Condon factors for the ionization transitions., ((c) 2004 American Institute of Physics.)

The dissociation of energy-selected ND(3) (+) to form ND(2) (+)+D near its threshold has been investigated using the pulsed field ionization-photoelectron (PFI-PE)-photoion coincidence method. The breakdown curves for ND(3) (+) and ND(2) (+) give a value of 15.891+/-0.001 eV for the 0 K dissociation threshold or appearance energy (AE) for ND(2) (+) from ND(3). We have also measured the PFI-PE vibrational bands for ND(3) (+)(X;v(2) (+)=0, 1, 2, and 3), revealing partially resolved rotational structures. The simulation of these bands yields precise ionization energies (IEs) for ND(3) (+) X(0,v(2) (+)=0-3,0,0)<--ND(3) X(0,0,0,0). Using the 0 K AE (ND(2) (+)) and IE(ND(3))=10.200+/-0.001 eV determined in the present study, together with the known 0 K bond dissociation energy for ND(3) [D(0)(D-ND(2))=4.7126+/-0.0025 eV], we have determined the D(0)(ND(2) (+)-D), IE(ND(2)), and 0 K heat of formation for ND(2) (+) to be 5.691+/-0.001 eV, 11.1784+/-0.0025 eV, and 1261.82+/-0.4 kJ/mol, respectively. The PFI-PE spectrum is found to exhibit a steplike feature near the AE(ND(2) (+)), indicating that the dissociation of excited ND(3) (+) at energies slightly above the dissociation threshold is prompt, occurring in the time scale