Two-color Resonant Two-photon Ionization Research Articles

Spectroscopic Investigation of cis-2,4-Difluorophenol Cation by Mass-analyzed Threshold Ionization Spectroscopy

We applied the two-color resonant two-photon ionization and mass-analyzed threshold ionization techniques to record the vibronic and cation spectra of 2,4-difluorophenol. As supported by our theoretical calculations, only the cis form of 2,4-difluorophenol involves in the two-photon photoexcitation and pulsed field ionization processes. The band origin of the S1 ← S0 electronic transition of cis-2,4-difluorophenol appears at 35 647 ± 2 cm and the adiabatic ionization energy is determined to be 70 030 ± 5 cm, respectively. Most of the observed active vibrations in the electronically excited S1 and cationic ground D0 states mainly involve in-plane ring deformation vibrations. Comparing these data of cis-2,4-difluorophenol with those of phenol, cis-2fluorophenol, and 4-fluorophenol, we found that there is an additivity rule associated with the energy shift resulting from the additional fluorine substitution.

Excited-State Structure, Vibrations, and Nonradiative Relaxation of Jet-Cooled 5-Fluorocytosine

The S0 → S1 vibronic spectrum and S1 state nonradiative relaxation of jet-cooled keto-amino 5-fluorocytosine (5FCyt) are investigated by two-color resonant two-photon ionization spectroscopy at 0.3 and 0.05 cm(–1) resolution. The 0(0)(0) rotational band contour is polarized in-plane, implying that the electronic transition is (1)ππ*. The electronic transition dipole moment orientation and the changes of rotational constants agree closely with the SCS-CC2 calculated values for the (1)ππ* (S1) transition of 5FCyt. The spectral region from 0 to 300 cm(–1) is dominated by overtone and combination bands of the out-of-plane ν1′ (boat), ν2′ (butterfly), and ν3′ (HN–C6H twist) vibrations, implying that the pyrimidinone frame is distorted out-of-plane by the (1)ππ* excitation, in agreement with SCS-CC2 calculations. The number of vibronic bands rises strongly around +350 cm(–1); this is attributed to the (1)ππ* state barrier to planarity that corresponds to the central maximum of the double-minimum out-of-plane vibrational potentials along the ν1′, ν2′, and ν3′ coordinates, which gives rise to a high density of vibronic excitations. At +1200 cm(–1), rapid nonradiative relaxation (k(nr) ≥ 10(12) s(–1)) sets in, which we interpret as the height of the (1)ππ* state barrier in front of the lowest S1/S0 conical intersection. This barrier in 5FCyt is 3 times higher than that in cytosine. The lifetimes of the ν′ = 0, 2ν1′, 2ν2′, 2ν1′ + 2ν2′, 4ν2′, and 2ν1′ + 4ν2′ levels are determined from Lorentzian widths fitted to the rotational band contours and are τ ≥ 75 ps for ν′ = 0, decreasing to τ ≥ 55 ps at the 2ν1′ + 4ν2′ level at +234 cm(–1). These gas-phase lifetimes are twice those of S1 state cytosine and 10–100 times those of the other canonical nucleobases in the gas phase. On the other hand, the 5FCyt gas-phase lifetime is close to the 73 ps lifetime in room-temperature solvents. This lack of dependence on temperature and on the surrounding medium implies that the 5FCyt nonradiative relaxation from its S1 ((1)ππ*) state is essentially controlled by the same ~1200 cm(–1) barrier and conical intersection both in the gas phase and in solution.

Low-lying excited states and nonradiative processes of 9-methyl-2-aminopurine.

The UV spectrum of the adenine analogue 9-methyl-2-aminopurine (9M-2AP) is investigated with one- and two-color resonant two-photon ionization spectroscopy at 0.3 and 0.05 cm(-1) resolution in a supersonic jet. The electronic origin at 32,252 cm(-1) exhibits methyl torsional subbands that originate from the 0A1'' (l = 0) and 1E(″) (l = ±1) torsional levels. These and further torsional bands that appear up to 00 (0)+230 cm(-1) allow to fit the threefold (V3) barriers of the torsional potentials as |V3''|=50 cm(-1) in the S0 and |V3'|=126 cm(-1) in the S1 state. Using the B3LYP density functional and correlated approximate second-order coupled cluster CC2 methods, the methyl orientation is calculated to be symmetric relative to the 2AP plane in both states, with barriers of V3''=20 cm(-1) and V3'=115 cm(-1). The 00 (0) rotational band contour is 75% in-plane (a/b) polarized, characteristic for a dominantly long-axis (1)ππ(*) excitation. The residual 25% c-axis polarization may indicate coupling of the (1)ππ(*) to the close-lying (1)nπ(*) state, calculated at 4.00 and 4.01 eV with the CC2 method. However, the CC2 calculated (1)nπ oscillator strength is only 6% of that of the (1)ππ(*) transition. The (1)ππ(*) vibronic spectrum is very complex, showing about 40 bands within the lowest 500 cm(-1). The methyl torsion and the low-frequency out-of-plane ν1' and ν2' vibrations are strongly coupled in the (1)ππ(*) state. This gives rise to many torsion-vibration combination bands built on out-of-plane fundamentals, which are without precedence in the (1)ππ(*) spectrum of 9H-2-aminopurine [S. Lobsiger, R. K. Sinha, M. Trachsel, and S. Leutwyler, J. Chem. Phys. 134, 114307 (2011)]. From the Lorentzian broadening needed to fit the 00 (0) contour of 9M-2AP, the (1)ππ(*) lifetime is τ ⩾ 120 ps, reflecting a rapid nonradiative transition.

Laser ionization and spectroscopy of Cu in superfluid helium nanodroplets

Mass and optical spectroscopic methods are used for the analysis of copper (Cu) atoms and clusters doped to helium nanodroplets (HeN). A two-color resonant two-photon ionization scheme is applied to study the Cu 2P1/2,3/2∘←2S1/2 ground state transition. The absorption is strongly broadened for Cu atoms submerged inside helium nanodroplets and a comparison with computed literature values is provided. An observed ejection of the dopant from the droplet is triggered upon excitation, populating energetically lower states. The formation of Cun clusters up to Cu7 inside helium nanodroplets was observed by means of electron impact ionization mass spectroscopy.

Structure and Intermolecular Vibrations of Perylene·trans-1,2-Dichloroethene, a Weak Charge-Transfer Complex

The vibronic spectra of strong charge-transfer complexes are often congested or diffuse and therefore difficult to analyze. We present the spectra of the π-stacked complex perylene trans-1,2-dichloroethene, which is in the limit of weak charge transfer, the electronic excitation remaining largely confined to the perylene moiety. The complex is formed in a supersonic jet, and its S0 ↔ S1 spectra are investigated by two-color resonant two-photon ionization (2C-R2PI) and fluorescence spectroscopies. Under optimized conditions, vibrationally cold (T(vib) ≈ 9 K) and well resolved spectra are obtained. These are dominated by vibrational progressions in the “hindered-rotation” Rc intermolecular vibration with very low frequencies of 11 (S0) and 13 cm(–1) (S1). The intermolecular Tz stretch and the Ra and Rb bend vibrations are also observed. The normally symmetry-forbidden intramolecular 1a(u) “twisting” vibration of perylene also appears, showing that the π- stacking interaction deforms the perylene moiety, lowering its local symmetry from D2h to D2. We calculate the structure and vibrations of this complex using six different density functional theory (DFT) methods (CAM-B3LYP, BH&HLYP, B97-D3, ωB97X-D, M06, and M06-2X) and compare the results to those calculated by correlated wave function methods (SCS-MP2 and SCS-CC2). The structures and vibrational frequencies predicted with the CAM-B3LYP and BH&HLYP methods disagree with the other calculations and with experiment. The other four DFT and the ab initio methods all predict a π-stacked “centered” structure with nearly coplanar perylene and dichloroethene moieties and intermolecular binding energies of D(e) = −20.8 to −26.1 kJ/mol. The 000 band of the S0 → S1 transition is red-shifted by δν = −301 cm(–1) relative to that of perylene, implying that the D(e) increases by 3.6 kJ/mol or 15% upon electronic excitation. The intermolecular vibrational frequencies are assigned to the calculated Rc, Tz, Ra, and Rb vibrations by comparing to the observed/calculated frequencies and S0 ↔ S1 Franck–Condon factors. Of the three TD-DFT methods tested, the hybrid-meta-GGA functional M06-2X shows the best agreement with the experimental electronic transition energies, spectral shifts, and vibronic spectra, closely followed by the ωB97X-D functional, while the M06 functional gives inferior results.

REMPI and MATI spectroscopic study of selected cis and trans 3-chlorostyrene rotamers

The ab initio and DFT calculations predict two stable rotamers, cis and trans, for 3-chlorostyrene in each of the S0, S1 and D0 states. In the two-color resonant two-photon ionization (2C-R2PI) spectra, the band origins of S1←S0 electronic transition of cis and trans 3-chlorostyrene appears respectively at 33766±3 and 34061±3cm−1. The mass-analyzed threshold ionization (MATI) spectra give the adiabatic ionization energies of 69701±4cm−1 and 69571±4cm−1 for cis and trans rotamers. Within the experimental detection limit, the measured transition energies are the same for both the 35Cl and 37Cl isotopomers. The observed active modes of rotamers in the S1 and D0 states involve mainly the in-plane ring deformation and substituent-sensitive bending vibration. It is derived from the 2C-R2PI and MATI spectroscopic data that the cis rotamer is more stable than the trans rotamer by 218±30, 513±30 and 88±30cm−1 for the S0, S1 and D0 states, respectively.

Jet-Cooled Spectroscopy of the α-Methylbenzyl Radical: Probing the State-Dependent Effects of Methyl Rocking Against a Radical Site

The state-dependent spectroscopy of α-methylbenzyl radical (α-MeBz) has been studied under jet-cooled conditions. Two-color resonant two-photon ionization (2C-R2PI), laser-induced fluorescence, and dispersed fluorescence spectra were obtained for the D0-D1 electronic transition of this prototypical resonance-stabilized radical in which the methyl group is immediately adjacent to the primary radical site. Extensive Franck-Condon activity in hindered rotor levels was observed in the excitation spectrum, reflecting a reorientation of the methyl group upon electronic excitation. Dispersed fluorescence spectra from the set of internal rotor levels are combined with the excitation spectrum to obtain a global fit of the barrier heights and angular change of the methyl group in both D0 and D1 states. The best-fit methyl rotor potential in the ground electronic state (D0) is a flat-topped 3-fold potential (V3" = 151 cm(-1), V6" = 34 cm(-1)) while the D1 state has a lower barrier (V3' = 72 cm(-1), V6' = 15 cm(-1)) with Δφ = ± π/3, π, consistent with a reorientation of the methyl group upon electronic excitation. The ground state results are compared with calculations carried out at the DFT B3LYP level of theory using the 6-311+G(d,p) basis set, and a variety of excited state calculations are carried out to compare against experiment. The preferred geometry of the methyl rotor in the ground state is anti, which switches to syn in the D1 state and in the cation. The calculations uncover a subtle combination of effects that contribute to the shift in orientation and change in barrier in the excited state relative to ground state. Steric interaction favors the anti conformation, while hyperconjugation is greater in the syn orientation. The presence of a second excited state close by D1 is postulated to influence the methyl rotor properties. A resonant ion-dip infrared (RIDIR) spectrum in the alkyl and aromatic CH stretch regions was also recorded, probing in a complementary way the state-dependent conformation of α-MeBz. Using a scheme in which infrared depletion occurs between excitation and ionization steps of the 2C-R2PI process, analogous infrared spectra in D1 were also obtained, probing the response of the CH stretch fundamentals to electronic excitation. A reduced-dimension Wilson G-matrix model was implemented to simulate and interpret the observed infrared results. Finally, photoionization efficiency scans were carried out to determine the adiabatic ionization threshold of α-MeBz (IP = 6.835 ± 0.002 eV) and provide thresholds for ionization out of specific internal rotor levels, which report on the methyl rotor barrier in the cation state.

Rotamers of m-chloroanisole studied by two-color resonant two-photon mass-analyzed threshold ionization spectroscopy

We apply the resonant two-photon ionization (R2PI) and mass-analyzed threshold ionization (MATI) techniques to record the vibronic and cation spectra of m-chloroanisole. The vibronic features appear in two series, built on 35,822±2 and 35,868±2cm−1, corresponding to the origins of the S1←S0 electronic transition (E1’s) of the two rotamers. Analysis of the MATI spectra gives the adiabatic ionization energies (IEs) of 67,645±5 and 68,008±5cm−1 for these two isomeric species. Comparing these data with those of anisole, we find that the chlorine substitution at the meta position leads to a red shift in the E1 and a blue shift in the IE. The observed R2PI and MATI bands mainly result from the in-plane ring deformation and substituent-sensitive bending vibrations of these species in the electronically excited S1 and cationic ground D0 states.

Rotamers of 3,4-difluorophenol studied by two-color resonant two-photon mass-analyzed threshold ionization spectroscopy

Abstract We applied the resonant two-photon ionization and mass-analyzed threshold ionization techniques to record the vibrational spectra of 3,4-difluorophenol in the first electronically excited and the ionic ground states. The band origins of the S1 ← S0 electronic transition of the cis and trans rotamers are found to be 35,486 ± 2 and 35,704 ± 2 cm−1 and the adiabatic ionization energies are 70,016 ± 5 and 70,203 ± 5 cm−1, respectively. The distinct spectral features mainly result from the in-plane ring deformation and substituent-sensitive bending vibrations. Spectral analysis suggests that the molecular geometry and vibrational coordinates of the cation in the D0 state resemble those of the neutral species in the S1 state for both cis and trans rotamers.

Excitonic splitting and vibronic coupling in 1,2-diphenoxyethane: Conformation-specific effects in the weak coupling limit

Vibrationally and rotationally resolved electronic spectra of 1,2-diphenoxyethane (C6H5-O-CH2-CH2-O-C6H5, DPOE) are reported for the isolated molecule under jet-cooled conditions. The spectra demonstrate that the two excited surfaces are within a few cm(-1) of one another over significant regions of the torsional potential energy surfaces that modulate the position and orientation of the two aromatic rings with respect to one another. Two-color resonant two-photon ionization (2C-R2PI) and laser-induced fluorescence excitation spectra were recorded in the near-ultraviolet in the region of the close-lying S0-S1 and S0-S2 states (36,400-36,750 cm(-1)). In previous work, double resonance spectroscopy in the ultraviolet and alkyl CH stretch regions of the infrared was used to identify and assign transitions to two conformational isomers differing primarily in the central C-C dihedral angle, a tgt conformation with C2 symmetry and a ttt conformation with C2h symmetry [E. G. Buchanan, E. L. Sibert, and T. S. Zwier, J. Phys. Chem. A 117, 2800 (2013)]. Comparison of 2C-R2PI spectra recorded in the m∕z 214 (all (12)C) and m∕z 215 (one (13)C) mass channels demonstrate the close proximity of the S1 and S2 excited states for both conformations, with an upper bound of 4 cm(-1) between them. High resolution spectra of the origin band of the tgt conformer reveal it to consist of two transitions at 36,422.91 and 36,423.93 cm(-1), with transition dipole moments perpendicular to one another. These are assigned to the S0-S1 and S0-S2 origin transitions with excited states of A and B symmetry, respectively, and an excitonic splitting of only 1.02 cm(-1). The excited state rotational constants and transition dipole coupling model directions prove that the electronic excitation is delocalized over the two rings. The ttt conformer has only one dipole-allowed electronic transition (Ag→Bu) giving rise to a pure b-type band at 36,508.77 cm(-1). Here, the asymmetry induced by a single (13)C atom in one of the rings is sufficient to localize the electronic excitation in one or the other ring. Dispersed fluorescence (DFL) spectra are used to provide assignments for all vibronic structure in the first 200 cm(-1)of both conformers. In the tgt conformer, both "a" and "b" symmetry fundamentals are observed, consistent with extensive vibronic coupling between the two dipole-allowed, nearly degenerate excited states. In the ttt conformer, the lowest frequency vibronic transition located 46 cm(-1) above the Bu origin is assigned to a bu fundamental (labeled R[overline]) built off the dipole-forbidden Ag state origin. The DFL spectrum of the Ag(R[overline](1)) level contains strong transitions to v(")(R[overline]) = 0, 1, and 2, seemingly at odds with vibronic coupling models. Studies of the DFL spectrum of this band as a function of distance from the nozzle reveal that much of the intensity in v(") = 1 arises from collisions of DPOE while in the excited state Ag(vb' = 1) level with He, producing Bu(R[overline] = 1) levels with large collision cross section. The remaining intensity in the fundamental at large x∕D is ascribed to emission from the (13)C isotopomer, for which this emission is dipole-allowed.

Excited-State Structure and Dynamics of Keto–Amino Cytosine: The 1ππ* State Is Nonplanar and Its Radiationless Decay Is Not Ultrafast

We have measured the mass- and tautomer-specific S0 → S1 vibronic spectra and S1 state lifetimes of the keto–amino tautomer of cytosine cooled in supersonic jets, using two-color resonant two-photon ionization (R2PI) spectroscopy at 0.05 cm(–1) resolution. The rotational contours of the 0(0)(0) band and nine vibronic bands up to +437 cm(–1) are polarized in the pyrimidinone plane, proving that the electronic excitation is to a 1ππ* state. All vibronic excitations up to +437 cm(–1) are overtone and combination bands of the low-frequency out-of-plane ν1′ (butterfly), ν2′ (boat), and ν3′ (H–N–C6–H twist) vibrations. UV vibronic spectrum simulations based on approximate second-order coupled-cluster (CC2) calculations of the ground and 1ππ* states are in good agreement with the experimental R2PI spectrum, but only if the calculated ν1′ and ν2′ frequencies are reduced by a factor of 4 and anharmonicity is included. Together with the high intensity of the ν1′ and ν2′ overtone vibronic excitations, this implies that the 1ππ* potential energy surface is much softer and much more anharmonic in the out-of-plane directions than predicted by the CC2 method. The 1ππ* state lifetime is determined from the Lorentzian line broadening necessary to reproduce the rotational band contours: at the 0(0)(0) band it is τ = 44 ps, remains at τ = 35–45 ps up to +205 cm(–1), and decreases to 25–30 ps up to +437 cm(–1). These lifetimes are 20–40 times longer than the 0.5–1.5 ps lifetimes previously measured with femtosecond pump–probe techniques at higher vibrational energies (1500–3800 cm(–1)). Thus, the nonradiative relaxation rate of keto–amino cytosine close to the 1ππ* state minimum is k(nr) 2.5 × 10(10) s(–1), much smaller than at higher energies. An additional nonradiative decay channel opens at +500 cm(–1) excess energy. Since high overtone bands of ν1′ and ν2′ are observed in the R2PI spectrum but only a single weak 2ν3′ band, we propose that ν3′ is a promoting mode for nonradiative decay, consistent with the observation that the ν3′ normal-mode eigenvector points toward the “C6-puckered” conical intersection geometry.

Mass-analyzed threshold ionization spectroscopy of trans-1-methoxynaphthalene cation and the methoxyl substitution effect

We applied the two-color resonant two-photon mass-analyzed threshold ionization technique to record the cation spectra of trans-1-methoxynaphthalene via five intermediate levels. The adiabatic ionization energy of this molecule is determined to be 61357±5cm−1, which is smaller than that of naphthalene by 4330cm−1. This indicates that the interaction between the OCH3 substituent and the aromatic ring is stronger in the cationic D0 state than that in the neutral S0 state. Comparing these data of trans-1-methoxynaphthalene with those of naphthalene, we find that the frequency difference of each mode depends on the vibrational pattern and the OCH3 group involved in the overall molecular motion.

Active vibrations of 1-cyanonaphthalene cation studied by mass-analyzed threshold ionization spectroscopy

We apply the two-color resonant two-photon mass-analyzed threshold ionization (MATI) spectroscopic technique to record the cation spectra of 1-cyanonaphthalene via four intermediate vibronic levels. The adiabatic ionization energy is determined to be 69 466±5cm−1. The distinct bands at 416, 472, 516, 669, and 852cm−1 result from in-plane ring deformation vibrations of the cation. Analysis of these MATI spectra suggests that the molecular geometry and vibrational coordinates of the observed vibrations of the cation in the ground D0 state resemble those of the neutral in the electronically excited S1 state.

Excitonic splitting and coherent electronic energy transfer in the gas-phase benzoic acid dimer

The benzoic acid dimer, (BZA)(2), is a paradigmatic symmetric hydrogen bonded dimer with two strong antiparallel hydrogen bonds. The excitonic S(1)/S(2) state splitting and coherent electronic energy transfer within supersonically cooled (BZA)(2) and its (13)C-, d(1) -, d(2) -, and (13)C/d(1) - isotopomers have been investigated by mass-resolved two-color resonant two-photon ionization spectroscopy. The (BZA)(2)-(h - h) and (BZA)(2)-(d - d) dimers are C(2h) symmetric, hence only the S(2) ← S(0) transition can be observed, the S(1) ← S(0) transition being strictly electric-dipole forbidden. A single (12)C/(13)C or H/D isotopic substitution reduces the symmetry of the dimer to C(s), so that the isotopic heterodimers (BZA)(2) - (13)C, (BZA)(2) -(h - d), (BZA)(2) -(h(13)C-d), and (BZA)(2) -(h - d(13)C) show both S(1) ← S(0) and S(2) ← S(0) bands. The S(1)/S(2) exciton splitting inferred is Δ(exc) = 0.94 ± 0.1 cm(-1). This is the smallest splitting observed so far for any H-bonded gas-phase dimer. Additional isotope-dependent contributions to the splittings, Δ(iso), arise from the change of the zero-point vibrational energy upon electronic excitation and range from Δ(iso) = 3.3 cm(-1) upon (12)C/(13)C substitution to 14.8 cm(-1) for carboxy H/D substitution. The degree of excitonic localization/delocalization can be sensitively measured via the relative intensities of the S(1) ← S(0) and S(2) ← S(0) origin bands; near-complete localization is observed even for a single (12)C/(13)C substitution. The S(1)/ S(2) energy gap of (BZA)(2) is Δ(calc) (exc)=11 cm(-1) when calculated by the approximate second-order perturbation theory (CC2) method. Upon correction for vibronic quenching, this decreases to Δ(vibron) (exc)=2.1 cm(-1) [P. Ottiger et al., J. Chem. Phys. 136, 174308 (2012)], in good agreement with the observed Δ(exc) = 0.94 cm(-1). The observed excitonic splittings can be converted to exciton hopping times τ(exc). For the (BZA)(2)-(h - h) homodimer τ(exc) = 18 ps, which is nearly 40 times shorter than the double proton transfer time of (BZA)(2) in its excited state [Kalkman et al., ChemPhysChem 9, 1788 (2008)]. Thus, the electronic energy transfer is much faster than the proton-transfer in (BZA)(2)(∗).

Excitation Spectra of the Jet-Cooled 4-Phenylbenzyl and 4-(4′-Methylphenyl)benzyl Radicals

The excitation spectra of jet-cooled 4-phenylbenzyl and 4-(4'-methylphenyl)benzyl radicals have been identified by a combination of resonant two-color two-photon ionization mass spectrometry and quantum chemical methods. Both radicals exhibit progressions in the biphenyl torsional mode, peaking near ν = 17. The lowest observed peak for 4-phenylbenzyl was observed at 18598 cm(-1) and is estimated to be the ν = 3 of the progression, while the lowest observed peak for the 4-(4'-methylphenyl)benzyl radical was observed at 18183 cm(-1) and is possibly the origin. The spectra are discussed and compared to other biphenyl and benzyl chromophores.

Rotamers of 3,4-difluoroanisole studied by two-color resonant two-photon mass-analyzed threshold ionization spectroscopy

We reported the vibronic and cation spectra of 3,4-difluoroanisole. The band origins of the S1←S0 electronic transition of the cis and trans rotamers appear at 35,505±2 and 35,711±2cm−1 and the adiabatic ionization energies are determined to be 67,780±5 and 68,125±5cm−1, respectively. Most of the observed active vibrations in the electronically excited S1 and cationic ground D0 states mainly involve in-plane ring deformation and substituent-sensitive bending vibrations. Analysis of the obtained vibronic and cation spectra suggests that the molecular geometry, symmetry, and vibrational coordinates of the cation in the D0 state are like those of the neutral species in the S1 state for these two isomeric species.

Out-of-Plane Low-Frequency Vibrations and Nonradiative Decay in the 1ππ* State of Jet-Cooled 5-Methylcytosine

We investigate the UV vibronic spectrum and excited-state nonradiative processes of jet-cooled 5-methylcytosine (5MCyt) using two-color resonant two-photon ionization spectroscopy at 0.3 and 0.05 cm(–1) resolution. In contrast to cytosine, which shows only five bands above its electronic origin, the lowest electronic transition of 5MCyt exhibits about 25 low-frequency vibronic bands that extend to 0(0)(0) + 450 cm(–1), allowing to extract detailed information on the excited-state electronic and nuclear structure. Most bands are overtones and combinations of the out-of-plane vibrations ν'(1), ν'(2), and ν'(3). Their large intensities reflect butterfly-, boat-, and twist-deformations of the 5MCyt framework upon electronic excitation. From the rotational contours of the 0(0)(0), 1(0)(2), 2(0)(2), and 3(0)(2) bands, the transition is found to be polarized along the in-plane a/b axes, characteristic of a (1)ππ* transition. Approximate second-order coupled-cluster (CC2) and time-dependent B3LYP calculations both predict that 5MCyt undergoes an out-of-plane deformation in its (1)ππ* (S(2)) state but both methods overestimate the out-of-plane ν'(1), ν'(2), and ν'(3) vibrational frequencies by a factor of 3–5. The TD-B3LYP (1)ππ* transition dipole moment direction is 10%:90% a:b, in good agreement with experiment. From the Lorentzian line shape contributions needed to fit the rotational contours, a lower limit to the 5MCyt (1)ππ* state lifetime at the 0(0)(0), 1(0)(2), 2(0)(2), and 3(0)(2) bands is determined as τ ≥ 30 ps. These values are in stark contrast to the ultrafast (picosecond) lifetimes measured for jet-cooled cytosine by femtosecond pump–probe techniques. They also confirm the observation from the R2PI spectrum that 5-methylation of cytosine increases its excited-state lifetime. The higher out-of-plane overtone and combination bands disappear from the spectrum by ~460 cm(–1), signaling the onset of lifetimes τ < 0.5 ps, induced by excitation of the ν'(1), ν'(2), and ν'(3) modes. Above 460 cm(–1) the in-plane fundamentals 9(0)(1), 10(0)(1), and 17(0)(1) are weakly observed, implying that the in-plane vibrations couple weakly to the out-of-plane modes and have lifetimes τ > 3 ps up to 720 cm(–1) excess vibrational energy.

Two-color resonant two-photon ionization and mass-analyzed threshold ionization spectroscopy of o-chloroanisole

Abstract We applied the two-color resonant two-photon ionization and mass-analyzed threshold ionization techniques to record the vibrationally resolved spectra of the selected isotopomers of o -chloroanisole in the electronically excited S 1 and cationic ground D 0 states. As supported by our theoretical calculations, only the trans form of o -chloroanisole involves in the two-photon photoexcitation and ionization processes. The band origins of the S 1 ← S 0 electronic transition and adiabatic ionization energies for the 35 Cl and 37 Cl isotopomers of o -chloroanisole are found to be the same within our detection limit, with the values of 35 745 ± 2 and 66 982 ± 5 cm −1 , respectively. The general spectral features of the two isotopomers are nearly identical and result from in-plane ring deformation and substituent-sensitive bending and stretching as well as the CH 3 torsional motions.

The electronic spectrum of Si3 I: Triplet D3h system

We report the measurement of a jet-cooled electronic spectrum of the silicon trimer. Si(3) was produced in a pulsed discharge of silane in argon, and the excitation spectrum examined in the 18 000-20 800 cm(-1) region. A combination of resonant two-color two-photon ionization (R2C2PI) time-of-flight mass spectroscopy, laser-induced fluorescence/dispersed fluorescence, and equation-of-motion coupled-cluster calculations have been used to establish that the observed spectrum is dominated by the 1(3)A(1)" - ã (3)A(2)' transition of the D(3h) isomer. The spectrum has an origin transition at 18,600 ± 4 cm(-1) and a short progression in the symmetric stretch with a frequency of ∼445 cm(-1), in good agreement with a predicted vertical transition energy of 2.34 eV for excitation to the 1(3)A(1)" state, which has a calculated symmetric stretching frequency of 480 cm(-1). In addition, a ∼505 cm(-1) ground state vibrational frequency determined from sequence bands and dispersed fluorescence is in agreement with an earlier zero-electron kinetic energy study of the lowest D(3h) state and with theory. A weaker, overlapping band system with a ∼360 cm(-1) progression, observed in the same mass channel (m/z = 84) by R2C2PI but under different discharge conditions, is thought to be due to transitions from the (more complicated) singlet C(2v) ground state ((1)A(1)) state of Si(3). Evidence of emission to this latter state in the triplet dispersed fluorescence spectra suggests extensive mixing in the excited triplet and singlet manifolds. Prospects for further spectroscopic characterization of the singlet system and direct measurement of the energy separation between the lowest singlet and triplet states are discussed.

Rotamers of m-fluoroanisole studied by two-color resonant two-photon mass-analyzed threshold ionization spectroscopy

We report the cation spectra of the cis and trans rotamers of m-fluoroanisole, recorded by using the two-color resonant two-photon mass-analyzed threshold ionization spectroscopic technique. The adiabatic ionization energies of the cis and trans rotamers are 67867±5 and 68304±5cm−1, respectively. The observed active vibrations 6b, 6a, 1, and 12 are found to have frequencies of 473, 549, 714, and 955cm−1 for the cis rotamer and 463, 565, 714, and 962cm−1 for the trans rotamer of m-fluoroanisole in the cationic ground state, respectively. These results show that the frequency depends on the nature, vibrational pattern, relative orientation and the extent of the F and OCH3 substituents participation in the normal vibration.