Direct identification of propargyl radical in combustion flames by vacuum ultraviolet photoionization mass spectrometry

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 (C(3)H(3)) 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 C(3)H(3) thus obtained is found to exhibit pronounced autoionization features, which are tentatively assigned as members of two vibrational progressions of C(3)H(3) in excited autoionizing Rydberg states. The ionization energy (IE = 8.674 +/- 0.001 eV) of C(3)H(3) 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 C(3)H(3) (+)(X;nu(i),i = 1-12)<--C(3)H(3)(X). The comparison between the pattern of FCFs and the autoionization peaks resolved in the VUV-PIE spectrum of C(3)H(3) 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(C(3)H(3)) suggests that the PIE spectra obtained in flames might have contributions from the photoionization of vibrationally excited C(3)H(3) and/or the dissociative photoionization processes involving larger hydrocarbon species formed in flames.     

Ionization energy measurements and electronic spectra for ThO

The ionization energy (IE) for ThO has been determined using photoionization efficiency and mass-analyzed threshold ionization measurements. An IE of 6.6038(12) eV was obtained, which was appreciably higher than the result from previous appearance potential measurements [6.1(1) eV]. The revised IE is 0.3 eV greater than that of atomic Th, indicating that neutral ThO is more tightly bound than ThO(+). The one-color two-photon resonant ionization spectrum of ThO was examined in the range of 315-370 nm. Rotationally resolved bands were recorded for three new electronic states (designated as E('),F('), and G(')). In addition, transitions to the A(')(v=1,2,3) levels and the N(v=2) level were observed for the first time. Ligand field theory predictions [L. A. Kaledin, J. E. McCord, and M. C. Heaven, J. Mol. Spectrosc. 164, 27 (1994)] were used to propose configurational assignments for 20 electronically excited states.     

Electronic and cationic states of 2-methylpropene (isobutene) studied by VUV absorption,scattered electron spectroscopy and AB initio multireference configuration interaction calculations

VUV (6.2–9 eV) and electron scattering spectra (1–9 eV) have been recorded for 2-methylpropene (isobutene). Also, electronic states of the molecule, including the ground state and cationic states, have been investigated using ab initio multi-reference configuration interaction calculations. Some Koopmans-type in the UV photoelectron spectrum are reassigned and a number of shake-up states computed. In the electronic spectrum, Rydberg excited have been assigned and a second valence excited state (σ π*) located within about 1 eV of the V(ππ*) state. The experiments show, and theory confirms, that the Rydberg R(π3s) state has a positive electron affinity. Some interesting correlations between ionisation energies, energies of shake-up state electronic excitation energies are identified.     

Quantum chemical investigation of the electronic spectra of the keto, enol, and keto-imine tautomers of cytosine

The low-lying excited singlet states of the keto, enol, and keto-imine tautomers of cytosine have been investigated employing a combined density functional/multireference configuration interaction (DFT/MRCI) method. Unconstrained geometry optimizations have yielded out-of-plain distorted structures of the pi --> pi and n --> pi excited states of all cytosine forms. For the keto tautomer, the DFT/MRCI adiabatic excitation energy of the pi --> pi state (4.06 eV including zero-point vibrational energy corrections) supports the resonant two-photon ionization (R2PI) spectrum (Nir et al. Phys. Chem. Chem. Phys. 2002, 5, 4780). On its S1 potential energy surface, a conical intersection between the 1pipi state and the electronic ground state has been identified. The barrier height of the reaction along a constrained minimum energy path amounts to merely 0.2 eV above the origin and explains the break-off of the R2PI spectrum. The 1pipi minimum of the enol tautomer is found at considerably higher excitation energies (4.50 eV). Because of significant geometry shifts with respect to the ground state, long vibrational progressions are expected, in accord with experimental observations. For the keto-imine tautomer, a crossing of the 1pipi potential energy surface with the ground-state surface has been found, too. Its n --> pi minimum (3.27 eV) is located well below the conical intersection between the pi --> pi and S0 states, but it will be difficult to observe because of its small transition moment. The identified conical intersections of the pi --> pi excited states of the keto cytosine tautomers are made responsible for the ultrafast decay to the electronic ground states and thus may explain their subpicoseconds lifetimes.     

Competition between photodissociation and photoionization in methyl iodide

CH₃I has a number of 5pπ → 6p Rydberg states which can be excited by two-photon absorption from the ground state. These two-photon absorptions have been previously detected by molecular ionization with a third photon. They can also be studied by observation of the VUV fluorescence from neutral photofragments (excited iodine atoms) following absorption of a third photon. Similarly it is found that absorption of one photon at 118 nm (10.5 eV) is followed by dissociation into excited iodine atoms as well as ionization. Iodine atom VUV emission is 2–10 times stronger from CD₃I than from CH₃I.     

Conical intersections in thymine

The mechanisms which are responsible for the radiationless deactivation of the npi* and pipi* excited singlet states of thymine have been investigated with multireference ab initio methods (the complete-active-space self-consistent-field (CASSCF) method and second-order perturbation theory with respect to the CASSCF reference (CASPT2)) as well as with the CC2 (approximated singles and doubles coupled-cluster) method. The vertical excitation energies, the equilibrium geometries of the 1npi*and 1pipi* states, as well as their adiabatic excitation energies have been determined. Three conical intersections of the S1 and S0 energy surfaces have been located. The energy profiles of the excited states and the ground state have been calculated with the CASSCF method along straight-line reaction paths leading from the ground-state equilibrium geometry to the conical intersections. All three conical intersections are characterized by strongly out-of-plane distorted geometries. The lowest-energy conical intersection (CI1) arises from a crossing of the lowest 1pipi* state with the electronic ground state. It is found to be accessible in a barrierless manner from the minimum of the 1pipi* state, providing a direct and fast pathway for the quenching of the population of the lowest optically allowed excited states of thymine. This result explains the complete diffuseness of the absorption spectrum of thymine in supersonic jets. The lowest vibronic levels of the optically nearly dark 1npi* state are predicted to lie below CI1, explaining the experimental observation of a long-lived population of dark excited states in gas-phase thymine.     

Theoretical study on the excited states of psoralen compounds bonded to a thymine residue

Time-dependent density functional theory calculations have been performed for the excited states of psoralen, 5-methoxypsoralen, and 8-methoxypsoralen in systems and furan and pyrone monoadducts bonded to a thymine residue. The theoretical assignments to ultraviolet (UV) absorption spectra of isolated systems have been performed. The present calculations have clarified that the excitation energies of the first singlet excited (S1) state of monoadducts are blue-shifted compared with the isolated systems. It is shown that, in particular, the S1 excitation energy of the pyrone monoadduct is significantly blue-shifted and, therefore, the pyrone monoadduct is not excited by UV-A light (300-400 nm), which is used in the photochemotherapy.     

Environmental photochemistry of nitro-PAHs: direct observation of ultrafast intersystem crossing in 1-nitropyrene

Femtosecond broadband transient absorption experiments of 1-nitropyrene, a nitro-polycyclic aromatic hydrocarbon of environmental concern are presented in cyclohexane and hexane solutions. The transient absorption spectra show the presence of three species that are assigned to the Franck-Condon excited lowest singlet (S1) state, the structurally relaxed S1 state, and the lowest excited triplet state. The spectral changes at early times are interpreted in terms of conformational dynamics; primarily due to an ultrafast rotation of the nitro group in the S1 state. This excited state relaxation is followed by intersystem crossing with a time constant of 7 ps. CIS/6-31G(d,p) calculations predict planarization of the nitro-aromatic torsional angle as the major nuclear relaxation coordinate, from 32.8 degrees at the HF/6-31G(d,p) level of theory in the ground state (27.46 degrees at B3LYP/6-31++G(d,p)) to 0.07 degrees in the S1 state. Vertical excitation energies at the TDDFT/6-31++G(d,p) and TDDFT/IEFPCM/6-31++G(d,p) levels of theory predict a small energy gap (<0.12 eV) between the S1(pipi*) state and the third excited triplet state T3(npi*) in the gas phase and in cyclohexane, respectively. The small energy gap suggests a large spin-orbit coupling between the S1(pipi*) and T3(npi*) states, which explains the ultrafast intersystem crossing of 1-nitropyrene in nonpolar solvents.     

High-resolution absorption spectrum of jet-cooled CH3Cl between 70,000 and 85,000 cm(-1): new assignments

The absorption spectrum of jet-cooled CH(3)Cl was photographed from 165 to 117 nm (or 60,000 - 85,000 cm(-1), 7.5-10.5 eV) at a resolution limit of 0.0008 nm (0.3-0.6 cm(-1) or 0.04-0.08 meV). Even in the best structured region of the spectrum, from 70,000 to 85,000 cm(-1) (8.7-10.5 eV), observed bandwidths (full width at half maximum) are large, from 50 to 150 cm(-1). No rotational feature could be resolved. The spectrum is dominated by two strong bands near 9 eV, 140 nm, the D and E bands of Mulliken [J. Chem. Phys. 8, 382 (1940)] or the spectral region D of Price [J. Chem. Phys.4, 539 (1936)]. Their relative intensity is incompatible with previous assignments, namely, to a triplet and a singlet state belonging to the same configuration. On the basis of the present ab initio calculations, those bands are now assigned to two singlet states, the (1)A(1) and (1)E excited states resulting from the 2e(3)4pe Rydberg configuration. The present calculations also reveal that the two (1)E states issued from 2e(3)4sa(1) and 2e(3)4pa(1) are quasidegenerate and strongly mixed. They should be assigned to the two broad bands near 8 eV, 160 nm, the B and C bands of Mulliken and Price. Three vibrational modes are observed to be active: the CCl bond stretch nu(3)(a(1)), and the CH(3) umbrella and rocking vibrations, respectively, nu(2)(a(1)) and nu(6)(e). The fundamental frequencies deduced are well within the ranges defined by the corresponding values in the neutral and ion ground states. The possibility of a dynamical Jahn-Teller effect induced by the nu(6)(e) vibrational mode in the (1)E Rydberg states is discussed.     

Time-dependent density functional theory studies of the photoswitching of the two-photon absorption spectra in stilbene, metacyclophenadiene, and diarylethene chromophores

Photochromophores such as cis-stilbene (1a), metacyclophenadiene (2a), and the diarylethene 3a undergo photoinduced conrotatory opening and closing of a central bond and are currently being sought out as potential candidates for media within 3D optical information storage devices. Strong molecular two-photon absorption (inducing the reversible photoisomerization) is a necessary feature for this application due to the need for high 3D spatial resolution. Here, the one- and two-photon absorption (OPA and TPA) characteristics of the open- and closed-ring isomers of 1-3 have been investigated using time-dependent density functional theory. It was determined that the excited states populated by two-photon absorption were nearly 1 eV higher in energy than the lowest energy excited state populated by one-photon absorption. The electronic structures of the TPA and OPA accessed states were then compared utilizing natural transition orbital analysis. There, it was found that states excited by OPA had pipi* character about the C-C framework associated with the bond formation/scission of the central C-C bond. In contrast, the states populated by TPA have pipi* character along the C-C skeletal periphery, including phenyl excitations. It is postulated that these differences in excited state electronic structure may lead to reaction pathways alternative to photoisomerization about the central C-C bond, impacting the utility of these compounds as 3D information storage media.     

Photoelectron spectroscopy of higher bromine and iodine oxide anions: electron affinities and electronic structures of BrO(2,3) and IO(2-4) radicals

This report details a photoelectron spectroscopy (PES) and theoretical investigation of electron affinities (EAs) and electronic structures of several atmospherically relevant higher bromine and iodine oxide molecules in the gas phase. PES spectra of BrO(2)(-) and IO(2)(-) were recorded at 12 K and four photon energies--355 nm/3.496 eV, 266 nm/4.661 eV, 193 nm/6.424 eV, and 157 nm/7.867 eV--while BrO(3)(-), IO(3)(-), and IO(4)(-) were only studied at 193 and 157 nm due to their expected high electron binding energies. Spectral features corresponding to transitions from the anionic ground state to the ground and excited states of the neutral are unraveled and resolved for each species. The EAs of these bromine and iodine oxides are experimentally determined for the first time (except for IO(2)) to be 2.515 ± 0.010 (BrO(2)), 2.575 ± 0.010 (IO(2)), 4.60 ± 0.05 (BrO(3)), 4.70 ± 0.05 (IO(3)), and 6.05 ± 0.05 eV (IO(4)). Three low-lying excited states along with their respective excitation energies are obtained for BrO(2) [1.69 (A (2)B(2)), 1.79 (B (2)A(1)), 1.99 eV (C (2)A(2))], BrO(3) [0.7 (A (2)A(2)), 1.6 (B (2)E), 3.1 eV (C (2)E)], and IO(3) [0.60 (A (2)A(2)), 1.20 (B (2)E), ～3.0 eV (C (2)E)], whereas six excited states of IO(2) are determined along with their respective excitation energies of 1.63 (A (2)B(2)), 1.73 (B (2)A(1)), 1.83 (C (2)A(2)), 4.23 (D (2)A(1)), 4.63 (E (2)B(2)), and 5.23 eV (F (2)B(1)). Periodate (IO(4)(-)) possesses a very high electron binding energy. Only one excited state feature with 0.95 eV excitation energy is shown in the 157 nm spectrum. Accompanying theoretical calculations reveal structural changes from the anions to the neutrals, and the calculated EAs are in good agreement with experimentally determined values. Franck-Condon factors simulations nicely reproduce the observed vibrational progressions for BrO(2) and IO(2). The low-lying excited state information is compared with theoretical calculations and discussed with their atmospheric implications.     

Theoretical study toward understanding ultrafast internal conversion of excited 9H-adenine

The CASPT2/CASSCF method with the 6-311G basis set and an active space up to (14, 11) was used to explore the ultrafast internal conversion mechanism for excited 9H-adenine. Three minima, two transition states, and seven conical intersections were obtained to build up the two deactivation pathways for the internal conversion mechanism. Special efforts were made to explore the excited-state potential energy surfaces near the Franck-Condon region and determine the various barriers in the processes of deactivation. The barrier required from the 1pipi (1La) state to deactivate nonradiatively is found to be lower than that required from the 1pipi (1Lb) state. On 250 nm excitation, the 1pipi (1La) state is populated, and the transition from 1pipi (1La) to the lowest 1npi state involves very low barriers, which may account for the observed short (<50 fs) lifetime of the 1pipi excited state. The deactivation of the lowest 1npi state is required to overcome a barrier of 3.15 kcal/mol, which should be responsible for the 750 fs lifetime of the npi excited state. On 267 nm excitation, the vibrationally active 1pipi (1Lb) state is populated. Excitation at 277 nm prepares the 1pipi (1Lb) state without much excessive vibrational energy, which may be responsible for the observed >2 ps lifetime.     

Quantum chemical modeling of photoadsorption properties of the nitrogen-vacancy point defect in diamond

Quantum chemical calculations of geometric and electronic structure and vertical transition energies for several low-lying excited states of the neutral and negatively charged nitrogen-vacancy point defect in diamond (NV(0) and NV(-)) have been performed employing various theoretical methods and basis sets and using finite model NC(n)H(m) clusters. Unpaired electrons in the ground doublet state of NV(0) and triplet state of NV(-) are found to be localized mainly on three carbon atoms around the vacancy and the electronic density on the nitrogen and rest of C atoms is only weakly disturbed. The lowest excited states involve different electronic distributions on molecular orbitals localized close to the vacancy and their wave functions exhibit a strong multireference character with significant contributions from diffuse functions. CASSCF calculations underestimate excitation energies for the anionic defect and overestimate those for the neutral system. The inclusion of dynamic electronic correlation at the CASPT2 level leads to a reasonable agreement (within 0.25 eV) of the calculated transition energy to the lowest excited state with experiment for both systems. Several excited states for NV(-) are found in the energy range of 2-3 eV, but only for the 1(3)E and 5(3)E states the excitation probabilities from the ground state are significant, with the first absorption band calculated at approximately 1.9 eV and the second lying 0.8-1 eV higher in energy than the first one. For NV(0), we predict the following order of electronic states: 1(2)E (0.0), 1(2)A(2) (approximately 2.4 eV), 2(2)E (2.7-2.8 eV), 1(2)A(1), 3(2)E (approximately 3.2 eV and higher).     

Excited state electronic structures and dynamics of NOCl: a new potential function set, absorption spectrum, and photodissociation mechanism

A set of analytical potential energy surfaces (PESs) for six singlet excited states of NOCl are constructed based on multireference configuration interaction calculations. The total absorption cross section at the energy range of 2-7 eV is calculated by quantum dynamics calculations with the present PESs and transition dipole moments. The calculated absorption spectrum agrees well with the experiment. It is also found that the A band with the absorption maximum at 6.3 eV is attributed to the transition to the 4 1A' state, though the excitations to the 3 1A' and 3 1A" states contribute to the spectrum at the energy range between 4 and 5 eV. The spin-forbidden transitions are concluded to be negligibly weak. The mechanism of photodissociation reaction at the energy region corresponding to the A band is examined. The nonadiabatic transition rates from the 4 1A' state to lower singlet and triplet states are estimated by Fermi's golden rule, and the transitions to the 1 1A' and 3 1A' states induced by vibronic coupling are found to be the predominant dissociation pathways. The experimentally observed energy dependence of the recoil anisotropy of the fragments is discussed based on the calculated nonadiabatic transition rates.     

Spiro[4.4]nonatetraene and its Positive and Negative Radical Ions: Molectronic Structure Investigations

The electronic structure of spiro[4.4]nonatetraene 1 as well as that of its radical anion and cation were studied by different spectroscopies. The electron‐energy‐loss spectrum in the gas phase revealed the lowest triplet state at 2.98 eV and a group of three overlapping triplet states in the 4.5 – 5.0 eV range, as well as a number of valence and Rydberg singlet excited states. Electron‐impact excitation functions of pure vibrational and triplet states identified various states of the negative ion, in particular the ground state with an attachment energy of 0.8 eV, an excited state corresponding to a temporary electron attachment to the 2b₁ MO at an attachment energy of 2.7 eV, and a core excited state at 4.0 eV. Electronic‐absorption spectroscopy in cryogenic matrices revealed several states of the positive ion, in particular a richly structured first band at 1.27 eV, and the first electronic transition of the radical anion. Vibrations of the ground state of the cation were probed by IR spectroscopy in a cryogenic matrix. The results are discussed on the basis of density‐functional and CASSCF/CASPT2 quantum‐chemical calculations. In their various forms, the calculations successfully rationalized the triplet and the singlet (valence and Rydberg) excitation energies of the neutral molecule, the excitation energies of the radical cation, its IR spectrum, the vibrations excited in the first electronic absorption band, and the energies of the ground and the first excited states of the anion. The difference of the anion excitation energies in the gas and condensed phases was rationalized by a calculation of the Jahn‐Teller distortion of the anion ground state. Contrary to expectations based on a single‐configuration model for the electronic states of 1 , it is found that the gap between the first two excited states is different in the singlet and the triplet manifold. This finding can be traced to the different importance of configuration interaction in the two multiplicity manifolds.     

Photoionization of propargyl and bromopropargyl radicals: a threshold photoelectron spectroscopic study

In this Article, we present mass-selected threshold photoelectron spectra of propargyl as well as the 1- and 3-bromopropargyl radicals. The reactive intermediates were produced by flash pyrolysis of suitable precursors and ionized by VUV synchrotron radiation. The TPES of the propargyl radical was simulated using data from a recent high-level computational study. An ionization energy (IE) of 8.71 ± 0.02 eV was obtained, in excellent agreement with computations, but slightly above previous experimental IEs. The pyrolysis of 1,3-dibromopropyne delivers both 1- and 3-bromopropargyl radicals that can be distinguished by their different ionization energies (8.34 and 8.16 eV). To explain the vibrational structure, a Franck-Condon simulation was performed, based on DFT calculations, which can account for all major spectral features. Bromopropargyl photoionizes dissociatively beginning at around 10.1 eV. Cationic excited states of 1- and 3-bromopropargyl were tentatively identified. The dissociative photoionization of the precursor (1,3-dibromopropyne) was also examined, delivering an AE(0K) (C(3)H(2)Br(+)/C(3)H(2)Br(2)) of 10.6 eV.     

Study of acyl group migration by femtosecond transient absorption spectroscopy and computational chemistry

The primary photophysical and photochemical processes in the photochemistry of 1-acetoxy-2-methoxyanthraquinone (1a) were studied using femtosecond transient absorption spectroscopy. Excitation of 1a at 270 nm results in the population of a set of highly excited singlet states. Internal conversion to the lowest singlet npi* excited state, followed by an intramolecular vibrational energy redistribution (IVR) process, proceeds with a time constant of 150 +/- 90 fs. The 1npi* excited state undergoes very fast intersystem crossing (ISC, 11 +/- 1 ps) to form the lowest triplet pipi* excited state which contains excess vibrational energy. The vibrational cooling occurs somewhat faster (4 +/- 1 ps) than ISC. The primary photochemical process, migration of acetoxy group, proceeds on the triplet potential energy surface with a time constant of 220 +/- 30 ps. The transient absorption spectra of the lowest singlet and triplet excited states of 1a, as well as the triplet excited state of the product, 9-acetoxy-2-methoxy-1,10-anthraquinone (2a), were detected. The assignments of the transient absorption spectra were supported by time-dependent DFT calculations of the UV-vis spectra of the proposed intermediates. All of the stationary points for acyl group migration on the triplet and ground state singlet potential energy surfaces were localized, and the influence of the acyl group substitution on the rate constants of the photochemical and thermal processes was analyzed.     

Combined vacuum ultraviolet laser and synchrotron pulsed field ionization study of CH2BrCl

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.     

New view of the ICN A continuum using photoelectron spectroscopy of ICN-

Negative-ion photoelectron spectroscopy of ICN(-) (X??(2)Σ(+)) reveals transitions to the ground electronic state (X??(1)Σ(+)) of ICN as well as the first five excited states ((3)Π(2), (3)Π(1), Π(0(-) ) (3), Π(0(+) ) (3), and (1)Π(1)) that make up the ICN A continuum. By starting from the equilibrium geometry of the anion, photoelectron spectroscopy characterizes the electronic structure of ICN at an elongated I-C bond length of 2.65 A?. Because of this bond elongation, the lowest three excited states of ICN ((3)Π(2), (3)Π(1), and Π(0(-) ) (3)) are resolved for the first time in the photoelectron spectrum. In addition, the spectrum has a structured peak that arises from the frequently studied conical intersection between the Π(0(+) ) (3) and (1)Π(1) states. The assignment of the spectrum is aided by MR-SO-CISD calculations of the potential energy surfaces for the anion and neutral ICN electronic states, along with calculations of the vibrational levels supported by these states. Through thermochemical cycles involving spectrally narrow transitions to the excited states of ICN, we determine the electron affinity, EA(ICN), to be 1.34(5) (+0.04∕-0.02) eV and the anion dissociation energy, D(0)(X??(2)Σ(+) I-CN(-)), to be 0.83 (+0.04/-0.02) eV.