Resonance-enhanced Multiphoton Ionization Spectroscopy Research Articles

Excited-State Double-Proton Transfer in the 7-Azaindole Dimer in the Gas Phase. 3. Reaction Mechanism Studied by Picosecond Time-Resolved REMPI Spectroscopy

The excited-state double-proton-transfer (ESDPT) reaction in the jet-cooled 7-azaindole dimer (7AI2) has been investigated with picosecond time-resolved resonance-enhanced multiphoton ionization spectroscopy. The observed decay profiles of 7AI2 by exciting the origin and the intermolecular stretch fundamental in the S1 state are well reproduced by single-exponential functions with time constants of 1.9 +/- 0.9 ps and 860 +/- 300 fs, respectively. This result provides clear evidence of the concerted mechanism of ESDPT in 7AI2.

Ultrafast excited-state dynamics in photochromic N-salicylideneaniline studied by femtosecond time-resolved REMPI spectroscopy

Ultrafast processes in photoexcited N-salicylideneaniline have been investigated with femtosecond time-resolved resonance-enhanced multiphoton ionization spectroscopy. The ion signals via the S(1)(n,pi( *)) state of the enol form as well as the proton-transferred cis-keto form emerge within a few hundred femtoseconds after photoexcitation to the first S(1)(pi,pi( *)) state of the enol form. This reveals that two ultrafast processes, excited-state intramolecular proton transfer (ESIPT) reaction and an internal conversion (IC) to the S(1)(n,pi( *)) state, occur on a time scale less than a few hundred femtoseconds from the S(1)(pi,pi( *)) state of the enol form. The rise time of the transient corresponding to the production of the proton-transferred cis-keto form is within 750 fs when near the red edge of the absorption is excited, indicating that the ESIPT reaction occurs within 750 fs. The decay time of the S(1)(pi,pi( *)) state of the cis-keto form is 8.9 ps by exciting the enol form at 370 nm, but it dramatically decreases to be 1.5-1.6 ps for the excitation at 365-320 nm. The decrease in the decay time has been attributed to the opening of an efficient nonradiative channel; an IC from S(1)(pi,pi( *)) to S(1)(n,pi( *)) of the cis-keto form promotes the production of the trans-keto form as the final photochromic products. The two IC processes may provide opposite effect on the quantum yield of photochromic products: IC in the enol form may substantially reduce the quantum yield, but IC in the cis-keto form increase it.

A non-lensed fiber-optic resonance-enhanced multiphoton ionization probe.

We have developed a miniature fiber-optic probe with no focusing optics for in situ analysis of volatile organic compounds (VOCs). The probe uses an optical fiber to transmit a laser pulse to a vapor sample causing it to ionize adjacent to the fiber tip through a resonance-enhanced multiphoton ionization (REMPI) process. The distal end of the optical fiber is contained co-axially within 2-mm-inner-diameter stainless steel tubing that serves as an electrode. The electrode is biased at a high positive potential to collect electrons. The current generated is shown to be proportional over about two orders of magnitude to the concentration of the species ionized. Visible wavelength REMPI spectroscopy is used to determine probe sensitivities of 20 ppb (benzene) and 43 ppb (toluene). Designing the probe without focusing optics specifies an achromatic ionization region constant in size and position as the laser wavelength is scanned, which simplifies data collection and reduction. Focusing achromatic systems are discussed and the potential signal improvement is estimated.

High-lying Rydberg states of vinyl bromide studied by two-photon resonant ionization spectroscopy

High-lying Rydberg states of vinyl bromide at 70 000–79 500 cm −1 have been investigated using 2 + 1 resonance-enhanced multiphoton ionization (REMPI) spectroscopy. Seven Rydberg series, including one s, three p, two d, and one f, have been identified from fitting the term values of observed electronic states to Rydberg formula rendering the ionization energy of 79 194 ± 17 cm −1 (9.819 ± 0.002 eV). All of the seven series converge to the same ionization-energy limit, corresponding to the ground state of vinyl bromide cation. The adiabatic ionization energy of vinyl bromide determined from this study is in excellent agreement with the values reported recently by other methods.

Two-color two-photon REMPI and ZEKE photoelectron spectroscopy of jet-cooled 2-chloropyrimidine

We report studies of supersonically cooled 2-chloropyrimidine (2-ClP) using two-color resonantly enhanced multiphoton ionization (REMPI) and two-color zero kinetic energy (ZEKE) photoelectron spectroscopy. With the aid of ab initio and density functional calculations, vibrational modes of the first electronically excited state (S 1) of the neutral species and those of the ground state cation (D 0) have been assigned, and the adiabatic ionization potential has been determined to be 77 106 ± 4 cm −1 (9.5600 ± 0.0005 eV). These results will be discussed in comparison with pyrimidine and other related compounds. Although the rate of intersystem crossing is affected by halosubstitution, 2-ClP and pyrimidine have qualitatively similar photophysical behaviors.

Resonance-enhanced multiphoton ionization spectroscopy of CH 3 and CD 3. Two-photon absorption selection rules and rotational line strengths of the ν3- and ν4-active vibronic transitions

To explore the possibility of K-level resolved, 2+1 resonance-enhanced multiphoton ionization (REMPI) processes of the methyl radical, the two-photon absorption selection rules and rotational line strengths of the 3 0 1 and 4 0 1 vibronic bands of the transition |np 2A″ 2〉←| X ̃ 2A″ 2〉 ( n=3 or 4) were reported. Stringent selection rules, which were imposed upon these two-photon transitions, are the initial K″=3 p ( p=0,1,2,…), Δ K=±2, Δ U=±3, and Δ N=0,±1,±2 ( O, P, Q, R, and S branches). The previously assigned 2 2 2 vibronic band of the methyl radical should be studied by the REMPI with a better spectral resolution and analyzed by the newly derived two-photon absorption selection rules and rotational line strength formulas.

The effect of conformation on the ionization energetics of n-butylbenzene. I. A threshold ionization study

Conformational isomers of the aromatic hydrocarbon n-butylbenzene have been studied using two-color REMPI (resonance enhanced multiphoton ionization) and MATI (mass analyzed threshold ionization) spectroscopy to explore the effect of conformation on ionization dynamics. Gauche- and anti-cationic conformers were selectively produced by two-color excitation via the respective S1 origins. Adiabatic ionization potentials of the gauche- and anti-conformations were determined to be 70 148 and 69 955±5 cm−1, respectively. Analysis of the REMPI and MATI spectra allowed the determination of the S0 (38 cm−1), S1 (100 cm−1), and D0 (−155 cm−1) gauche- and anti-conformer energy differences. Spectral features and vibrational modes are interpreted with the aid of MP2/cc-pVDZ ab initio calculations, and ionization-induced changes in the molecular conformations discussed.

Investigation of some Rydberg states of ketene by two-photon resonance-enhanced multiphoton ionization spectroscopy

Rydberg states of CH2CO and CD2CO in the 54 000–72 000 cm−1 spectral range have been reinvestigated using two-photon resonance-enhanced multiphoton-ionization spectroscopy. Improved resolution and sensitivity has allowed for identifications of more Rydberg states than in previous work. Based on an analysis of rotational structures and quantum defects and a comparison with the results of theoretical calculations, transitions to the 3pz, 4py, 4pz, and 5py, three components of 4d, and two components of 4f Rydberg states with a ground-state ionic core (X̃ 2B1) are identified. Several transitions have been reassigned. Vibrational wave numbers indicate that the geometry of the [X̃ 2B1]3py(1A2) state is almost identical to that of the corresponding cation in its ground electronic state, with C2v symmetry, whereas that of the [X̃ 2B1]3px(1A1) state differs significantly from those of the neutral molecule and the cation in their ground states, consistent with previous quantum chemical calculations that indicated that the [X̃ 2B1]3px(1A1) state has Cs symmetry. The energy ordering of the three components of the 3p Rydberg states is found to be 3px<3py<3pz, in agreement with a previous theoretical prediction using the equation-of-motion coupled-cluster singles and doubles polarized basis set methods and a first-principles vibronic model simulation. Excitations of the vibrational modes of b1 (e.g., C=C=O out-of-plane bending, CH2 or CD2 wagging) and b2 symmetries (e.g., C=C=O in-plane bending) are observed in several Rydberg states of CH2CO and CD2CO.

One- and two-photon excitation vibronic spectra of 2-methylallyl radical at 4.6–5.6 eV

Vibronically excited 2-methylallyl radical [CH2C(CH3)CH2] at 4.6–5.6 eV has been studied by 1+1 and 2+2 resonance-enhanced multiphoton ionization (REMPI) spectroscopy. The 2-methylallyl radicals were produced by the flash pyrolysis of 3-bromo-2-methylpropene in a supersonic-jet expansion. The 2+2 REMPI spectrum of 2-methylallyl radical at 38 000–40 700 cm−1 is identified as B̃(1 2A1)←X̃(1 2A2) transition, i.e., the excitation of a nonbonding electron to the 3s Rydberg state (3s←n). Seven lowest-lying electronic states with excitation energy below 6 eV have been calculated in an MRCI level. Two new electronic bands have been observed at 38 500–41 000 cm−1 by 1+1 REMPI spectroscopy and assigned to C̃(1 2B2)←X̃(1 2A2) and Ẽ(2 2A2)←X̃(1 2A2). Much broader 1+1 REMPI signals at 41 000–43 5 00 cm−1 with HWHM of ∼80 cm−1 for each vibronic band could be due to D̃(2 2B2)←X̃(1 2A2) and/or F̃(3 2B2)←X̃(1 2A2) via an intensity borrowing from C̃(1 2B2)←X̃(1 2A2). Taking the computed geometries and vibrations of the ground- and excited electronic states, Franck–Condon factors (FCFs) have been calculated. Combining the FCFs with calculated excitation energies and oscillator strengths of the six electronic states at 4–6 eV, predicted spectral patterns have been used to assist spectroscopic analysis for the observed vibronic spectra of 2-methylallyl radical.

Near UV photodissociation of CD 3SCD 3: CD 3 fragment ( [formula omitted], [formula omitted]) vector correlations

The alignment of CD 3 fragments following the photodissociation of fully deuterated dimethyl sulfide (CD 3SCD 3) on the first absorption band (at 219 nm) has been characterised using (2 + 1) resonance-enhanced multiphoton ionization (REMPI) spectroscopy. Operation of the time-of-flight (TOF) mass spectrometer with a low ion extraction field rejects off-axis travelling CD 3 fragments, allowing velocity-selected REMPI spectra to be obtained. The strong dependence of the REMPI signal on the polarization of the probe laser is adequately explained by an impulsive model dissociation where the fragments recoil axially along the CD 3S–CD 3 bond. In general, the methyl fragments are found to be preferentially spinning about their C 3 axis after photodissociation, correlating with zero-point torsional vibrational motion of the parent molecule.

Dynamics of excited OCS investigated by resonance-enhanced multiphoton ionization

The predissociation decay behavior of molecule carbonyl sulfide (OCS) has been investigated by resonance-enhanced multiphoton ionization spectroscopy using the pump-probe technique of dichroic femtosecond lasers in real time. The lifetime of excited OCS around $74720 {\mathrm{cm}}^{\ensuremath{-}1}$ by two-photon absorption of 268 nm, corresponding to ${\ensuremath{\upsilon}}_{1}=1$ of the Rydberg state ${[}^{2}{\ensuremath{\Pi}}_{1/2}]4p\ensuremath{\pi}{(}^{1}{\ensuremath{\Sigma}}^{+}),$ is directly determined to be ${\ensuremath{\tau}}_{D}=1071\ifmmode\pm\else\textpm\fi{}11 \mathrm{fs}.$ This picosecond decay process indicates that the excited state is predissociative. The temporal information of dissociation enriches the knowledge of the potential-energy surface of the associative excited state.

Two-photon resonant ionization spectroscopy of the allyl-h5 and allyl-d5 radicals: Rydberg states and ionization energies

The vibronic spectra of allyl-h5 (CH2CHCH2) and allyl-d5 (CD2CDCD2) radicals at 6.0–8.2 eV have been observed using 2+1 resonance-enhanced multiphoton ionization (REMPI) spectroscopy. The allyl-h5 and allyl-d5 radicals were produced in the nozzle of a supersonic jet expansion by the pyrolysis of CH2CHCH2I and CD2CDCD2Br, respectively. Together with our previous study, Rydberg states of ns (n=4–12) for allyl-h5 and (n=4–10) for allyl-d5 have been observed in the REMPI spectra. Prominent vibrational progressions with the spacings of ∼440 cm−1 in allyl-h5 and ∼360 cm−1 in allyl-d5 are observed in the s Rydberg series. The totally symmetric ∠CCC-bending mode is responsible for the vibrational progressions. Facilitated with ab initio calculated Franck–Condon factors, other vibrations (CH2 rocking and C3 stretching) observed in the s Rydberg series have also been identified. The adiabatic ionization energies, 65 594±9 cm−1 (8.133±0.001 eV) for allyl-h5 and 65 527±16 cm−1 (8.124±0.002 eV) for allyl-d5, are extrapolated from the convergence of the observed ns Rydberg series.

REMPI temperature measurement in molecular beam sampled low-pressure flames

The cooling effect in molecular beam (MB) sampling from low-pressure flames and gas mixtures was investigated. Although the MB method is often used to study the flame structure of low-pressure flames, typically combined with mass spectrometric (MS) detection, it is poorly characterized. The temperature of the MB must be known, especially if species concentrations are to be measured with spectroscopic methods, like resonance-enhanced multiphoton ionization (REMPI) spectroscopy. In the present study, two independent MBMS instruments, which are very similar to those used previously by different groups, were investigated starting with pressures of 40 and 50 mbar in the burner chamber. The rotational temperatures of NO and benzene were determined using REMPI spectroscopy for different initial conditions: the ions were separated by time-of-flight mass spectrometers. Two REMPI excitation schemes were applied: for NO the first step was always the excitation of the A-X transition near 225 nm, while benzene was excited and ionized at wavelengths near 259 nm. Unexpectedly, molecular beams from cold-gas flows were cooled very slightly by 10%–25%. In the molecular beams derived from low-pressure flames, the cooling effect was stronger, with final rotational temperatures of 300–400 K, but the MB temperature was virtually independent of the initial temperature. A possible explanation of this finding would be that the cooling takes place to a large extent by wall collisions within the nozzle and to a lesser degree by intermolecular collisions.

Two-photon vibronic spectra of vinyl chloride at 7.3–10 eV

The vibronic spectra of vinyl chloride at 7.3–10 eV have been studied using 2+1 resonance-enhanced multiphoton ionization (REMPI) spectroscopy. The geometries and vibrational frequencies of vinyl chloride and its cation calculated with ab initio methods are utilized to compute the Franck–Condon factors. The transitions to the Rydberg states of π→ns, npσ, npπ, nd, nf, and nCl→3s are responsible for the observed REMPI spectra of vinyl chloride. The complex vibrational structures for the observed transitions are successfully identified with the aid of calculated Franck–Condon factors.

Multiphoton spectroscopy of NO?Rg (Rg = rare gas) van der Waals systems

The spectroscopic characterization of the electronic ground and excited state interactions of NO with different rare gases is reviewed. In its electronic ground state, the NO molecule continues to serve as one of the most important benchmark systems for investigating van der Waals interactions involving open shell molecules. Combining high resolution infrared excitation with resonance enhanced multiphoton ionization (REMPI) detection, bound intermolecular bend-stretch levels have been detected for the complexes NO-Ar and NO-Ne for the first time. In parallel with these new experimental advances, new high-level ab-initio potential energy surfaces have been calculated very recently for several NO-Rg (Rg = rare gas) systems. Predictions for the bound states based on these potentials are in excellent agreement with the high resolution infrared spectra for NO-Ar and NO- Ne, demonstrating the great accuracy achieved with current ab initio methods. The large number of excited electronic states of NO provides ideal opportunities for studying molecular interactions in different excited states. The low lying Rydberg states are especially interesting because the Rydberg electron can reside in an orbital whose size is comparable with or larger than the van der Waals radius of the complexed atom. The existing set of experimental data on Rydberg states with n = 3(l = 0, 1, 2) and n = 4(l = 0) are reviewed critically. The excited states have been investigated employing mainly REMPI spectroscopy. The data provide detailed information about the bound levels of the associated intermolecular surfaces. Most of the work has concentrated on the complexes with Ar and Ne. Additional data for the states Ã2

Electronic and infrared spectra of jet-cooled 4-aminobenzonitrile-H 2O. Change of NH 2 from proton acceptor to proton donor by CN substitution

The electronic and infrared spectra of jet-cooled 4-aminobenzonitrile–(H 2O) 1 (4ABN–(H 2O) 1) hydrogen-bonded complex have been measured by the resonance-enhanced multiphoton ionization (REMPI) and infrared-dip (IR-dip) spectroscopy. Both the amino and cyano groups form intermolecular hydrogen bond with water, providing two stable isomers. It has been found that the substitution of the CN group at the para-position of aniline changes the electronic nature of the amino group in the S 0 state from a proton acceptor to a proton donor.

Identification of the 3pπue′ 3Δu Rydberg state of O2 by (3+1) resonance-enhanced multiphoton-ionization spectroscopy

The diffuse v=0 to 4 levels of the 3pπu e′ 3Δu Rydberg state of O2 are identified and characterized using (3+1) resonance-enhanced multiphoton-ionization spectroscopy. An experimental potential-energy curve for the e′ state, together with the magnitude of its interaction with the A′ 3Δu valence state, are determined semiempirically by analyzing the predissociated spectra using the coupled-channel Schrödinger-equation method. The experimental Rydberg-valence coupling (930 cm−1) exceeds significantly the value calculated ab initio by Buenker and Peyerimhoff [Chem. Phys. Lett. 34, 225 (1975)] (<320 cm−1), but is in good agreement with the single-configuration model prediction of Lewis et al. [J. Chem. Phys. 113, 2214 (2000)] (900 cm−1).

Multiphoton ionization and photoelectron spectroscopy of 1,3-trans-butadiene via its 3dπ Rydberg state

Resonance-enhanced multiphoton ionization (REMPI) and photoelectron spectroscopy (PES), have been used to study the Ag1(3dπ) Rydberg state of 1,3-trans-butadiene in the two photon energy range from 61 000 to 66 400 cm−1. The Ag1(3dπ) spectrum is dominated by the ν4′, ν6′, and ν9′ vibrational modes, with some excitation of the ν8′ mode, as well. Photoelectron spectroscopy shows that the dominant ionization pathways are diagonal, i.e., they produce cations in the same vibrational level that was populated in the Rydberg state. Weaker off-diagonal ionization is also observed, with excitation of the ν4+, ν6+, and ν9+ modes. The relative intensities of diagonal and off-diagonal PES bands are observed to be strongly dependent on the angle between the laser polarization and the detection axis. It is possible to use REMPI to generate state-selected cations, however, the nascent ions are quite efficiently photodissociated by the REMPI laser.

Out-of-Plane Modes of cis-1,3,5-Hexatriene: Frequency Shifts in the 2A1 and 11B1 Excited States

The calculation of Hessian matrices for the out-of-plane modes of cis-1,3,5-hexatriene (CHT)in the 11A1, 21A1, and 11B1 states is reported at the equilibrium geometry of the electronic ground state. The complete-active-space self-consistent-field (CASSCF) electronic structure model is employed. π and σ orbitals are included in the active space to account for the mixing of both types of molecular orbitals at nonplanar conformations of CHT. On the basis of the Hessian matrices and the resulting approximate normal frequencies, an interpretation of recent spectroscopic evidence for the valence excited states of CHT is attempted. In particular, we make assignments of transitions in the low-frequency region of the hv + hv resonance enhanced multiphoton ionization (REMPI) spectrum of the 21A1 state. The predicted excited-state frequencies appear to be accurate and normal coordinate rotations induced by electronic excitation are shown to play an important role for the intensity distributions observed by REMPI and resonance Raman spectroscopy.

Observation of completely destructive quantum interference between interacting resonances in molecular predissociation.

A unique observation is presented of interacting predissociating resonances which exhibit completely destructive interference in a region between the resonances. The use of a double-resonance technique, in which single rotational levels of the b (1)Sigma(+)(g) state of O2, prepared by pumping the magnetic-dipole b <--X transition, are probed by (2+1) resonance-enhanced multiphoton-ionization spectroscopy, eliminates overlapping rotational structure and enables observation of the interference process. Using a diabatic coupled-channel model, the interacting resonances are shown to be derived from the d (1)Pi(g)(v = 3,J = 17) Rydberg and II (1)Delta(g)(v = 6,J = 17) valence states.