Two-color Resonance-enhanced Multiphoton Ionization Research Articles

Significantly Improved Detection of Molecular Oxygen by Two-Color Resonance-Enhanced Multiphoton Ionization.

We report a new spectroscopic detection scheme for molecular oxygen that achieves roughly two orders of magnitude higher sensitivity for fully rotationally resolved spectra than the current state of the art. Two-color (2 + 1') resonance-enhanced multiphoton ionization (REMPI) via the 3d Rydberg complex yields state-selective spectra with signal comparable to the intense but diffuse C 3sσ 3Πg ← X 3Σg- (2 + 1) REMPI bands without significant saturation or broadening. The resulting increase in sensitivity permitted observation of the very weak 3dπ 1Δ2 ← X 3Σg- transitions and is independent of the intermediate state. This advance in ionization efficiency and quantum state-selective sensitivity for O2 promises to aid physical and chemical studies across a wide variety of fields.

Gas-Phase Spectroscopy of Laser-Desorbed Acedan and Proline-Acedan

Acedan (2-acetyl-6-dialkylaminonaphthalene) and prolineacedan (A and B in Scheme 1, respectively) are precursors for two-photon fluorescent probes such as ANO1, which turns on its fluorescence when bound to nitric oxide (NO). In spite of their similar molecular structures, A and B have different photophysical properties including absorption coefficient and fluorescence quantum yield. In order to prove the hypothesis that their different fluorescence properties stem from their different conformational structures, we tried to measure their conformation-specific UV and IR spectra in gas phase. The experimental setup has been described in previous publications. In-house synthesized powder sample of A or B was mixed with carbon black powder and the mixture was deposited on the lateral face of a graphite disk. The sample was desorbed by 1064 nm radiation of a pulsed Nd:YAG laser in vacuum, and the desorbed molecules were subsequently cooled down by pulsed supersonic expansion of Ar gas at 40 bar. UV spectra were measured by two-color resonance-enhanced multiphoton ionization (REMPI) with frequency-doubled output of Nd:YAG pumped dye lasers and a time-of-flight mass spectrometer. IR laser was provided by difference frequency generation of a dye laser and an Nd:YAG laser. IR-UV spectroscopy was used to record conformer-specific IR spectrum while the IR was scanned, and conformer-specific UV spectrum while the UV was scanned. Figure 1 shows 1+1' REMPI spectra of B (m/z = 283) and its fragments (m/z = 182, 241) with the second UV at 266 nm. Even though laser-desorption was employed to vaporize the sample, significant fragmentation was observed in the photoionization mass spectrum. The fragment of B at m/z = 241 (241) shows much stronger ion signal than the molecular ion (B, m/z = 283). As B and 241 showed similar vibronic bands in this region, it was assumed that 241 is issued from dissociative ionization of B, and the strong REMPI spectrum of 241 was presumed to reflect that of B. Figure 2 shows the 1+1' REMPI spectrum of 241 at lower

Electronic spectroscopy of jet-cooled YbNH3

We report the first spectroscopic study of a complex consisting of a rare earth atom in combination with ammonia. Using two-color resonance-enhanced multiphoton ionization (REMPI) spectroscopy, the lowest energy electronic transition of YbNH(3) has been found in the near-infrared. The spectrum arises from a spin-forbidden transition between the (1)A(1) ground electronic state and the lowest (3)E excited electronic state. The transition is metal centered and approximately correlates with the Yb 6s6p (3)P ← 6s(2) (1)S transition. The observation of clear spin-orbit structure in the spectrum confirms the C(3v) symmetry of YbNH(3). Vibrational structure is also observed in the REMPI spectrum, which is dominated by excitation of the Yb-N stretching vibration.

Electronic spectroscopy of the Au(6p)–Kr complex

We report electronic absorption spectra, recorded using one- and two-color resonance-enhanced multiphoton ionization spectroscopy, of the Au-Kr complex. The transition is localized on the gold atom, and corresponds to a 6p<--6s atomic excitation; we observe transitions to the D (2)Pi(1/2) and D (2)Pi(3/2) spin-orbit states. In addition, we report the results of ab initio calculations, which consider electronic states arising from the 6 (2)S, 5 (2)D, and 6 (2)P atomic energy levels of Au. Further, we also report an accurate value for the dissociation energy of the ground state of Au-Kr, based on basis set extrapolated RCCSD(T) calculations. The experimental results are discussed in the light of the theoretical ones.

Electronic spectra of 7-azaindole/ammonia clusters and their photochemical reactivity

The S(1)-S(0) electronic spectra of 7-azaindole-(NH(3))(n) clusters (n=1-3) were measured by mass-selected two-color resonance-enhanced multiphoton ionization spectroscopy. The laser-induced fluorescence spectrum obtained by monitoring the UV fluorescence shows well-structured vibrational bands for the monomer and 7-azaindole-(NH(3))(1,2) clusters, while no signals appear for the 7-azaindole-(NH(3))(3) cluster. The action spectrum obtained by monitoring visible emission shows no signal for all species, which suggests little reactivity for excited-state proton/hydrogen transfer. From the observed and calculated IR spectra, the geometry of 7-azaindole-(NH(3))(1,2) was concluded to be a hydrogen-bonded bridge form, which is similar to the photochemically reactive 7-hydroxyquinoline-(NH(3))(3) cluster. The difference in the photochemical reactivity is discussed on the basis of excited-state quantum chemical calculations.

Effect of Noncovalent Interactions on the n-Butylbenzene···Ar Cluster Studied by Mass Analyzed Threshold Ionization Spectroscopy and ab initio Computations

Clusters of Ar bound to isomers of the aromatic hydrocarbon n-butylbenzene (BB) have been studied using two-color REMPI (resonance enhanced multiphoton ionization) and MATI (mass analyzed threshold ionization) spectroscopy to explore noncovalent vdW interactions between these two moieties. Blue shifts of excitation energy were observed for gauche-BB...Ar clusters, and red shifts for anti-BB...Ar clusters were observed. Adiabatic ionization energies (IEs) of the conformer BB-I...Ar and BB-V...Ar were determined as 70052 and 69845 +/- 5 cm (-1), respectively. Spectral features and vibrational modes were interpreted with the aid of UMP2/cc-pVDZ ab initio calculations. Data of complexation shifts of the alkyl-benzenes and their argon clusters were collected and discussed. Using the CCSD(T) method at complete basis set (CBS) level, interaction energies for the neutral ground states of BB-I...Ar and BB-V...Ar were obtained as 650 and 558 cm (-1), respectively. Combining the CBS calculation results and the REMPI and MATI spectra allowed further the determination of the interaction energies and the energetics of BB...Ar in the excited neutral S 1 and the D 0 cationic ground states.

Effect of Noncovalent Interactions on Conformers of the n-Butylbenzene Monomer Studied by Mass Analyzed Threshold Ionization Spectroscopy and Basis-set Convergent ab initio Computations

Two conformational isomers of the aromatic hydrocarbon n-butylbenzene have been studied using two-color MATI (mass analyzed threshold ionization) spectroscopy to explore the effect of conformation on ionization dynamics. Cationic states of g auche-conformer III and anti- conformers IV were selectively produced by two-color excitation via the respective S 1 origins. Adiabatic ionization potentials of the gauche- and anti-conformations were determined to be 70146 and 69872 +/- 5 cm (-1) respectively. 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 are discussed. Complete basis set (CBS) ab initio studies at MP2 level reveal reliable energetics for all four n-butylbenzene conformers observed in earlier two-color REMPI (resonance enhanced multiphoton ionization) spectra. For the S 0 state, the energies of conformer III, IV and V are above conformer I by 130, 289, 73 cm (-1), respectively. Furthermore, the combination of the CBS calculations with the measured REMPI, MATI spectra allowed the determination of the energetics of all four conformers in the S 1 and D 0 states.

Simultaneous Measurement of Nitric Oxide (NO) and Nitrogen Dioxide (NO2) in Simulated Automobile Exhaust Using Medium Pressure Ionization—Mass Spectrometry

In previous papers we have demonstrated two different, two-color resonance-enhanced multiphoton ionization (REMPI) schemes for the simultaneous measurement of trace amounts (ppbV to pptV) of nitrogen monoxide (NO) and nitrogen dioxide (NO(2)). The goal of this study is to provide a laser ionization-mass spectrometric scheme capable of measuring ppmV to ppthV concentrations of NO and NO(2) within vehicle exhaust containing up to ppthV of aromatic hydrocarbons and a time frame of seconds. Two ionization schemes are used here to measure NO and NO(2) in simulated automobile exhaust with three different sources. REMPI Scheme 1 uses broad-bandwidth light and an effusive source to measure NO (limit of detection (LOD) 300 ppmV), NO(2) (LOD 100 ppmV), and aromatic hydrocarbons (via photoionization) along with fragments (via electron impact). REMPI Scheme 2 uses narrow-bandwidth light and a medium pressure laser ionization (MPLI) source to measure NO (LOD 60 ppmV), NO(2) (LOD 3 ppmV), and fragments (via electron impact). The LOD is determined using 10-second sampling times. A newly developed delayed-ion extraction technique for MPLI is then applied to REMPI Scheme 2, dramatically reducing the electron impact signal, so that only NO and NO(2) are observed. We conclude that Scheme 2 with delayed-electron extraction is best suited for measuring in situ NO and NO(2) within engine exhaust.

Two-color resonance-enhanced multiphoton ionization study of the lowest Rydberg p state of bis(eta6-benzene)chromium and its deuterated derivatives.

Two-color resonance-enhanced multiphoton ionization (REMPI) spectra of jet-cooled (eta(6)-C(6)H(6))(2)Cr(1), (eta(6)-C(6)D(6))(2)Cr(2), and (eta(6)-C(6)D(6))(eta(6)-C(6)D(5)H)Cr(3) have been measured with use of the 3d(z)2-->R4p(x,y) Rydberg transition as the first step of the electronic excitation. The 0(0) (0) Rydberg component shifts by 59 and 54 cm(-1) to red when one goes from 1 to 2 and 3, respectively. Surprisingly, the REMPI spectra of 1-3 show very rich vibronic structures revealing both totally symmetric vibrations and degenerate vibrational modes. Presence of intense peaks corresponding to the e(2g) modes in the spectra of 1 and 2 is indicative of Jahn-Teller coupling in the R4p(x,y) Rydberg state. Additional REMPI resonances appear on going from 1 and 2 to 3 as a result of the symmetry reduction. The vibronic components in the spectra of 1-3 were assigned on the basis of the selection rules and comparison with the vibrational frequencies of the 1 and 2 ground-state molecules. The frequencies of over 10 normal vibrations have been determined for the gas-phase 1-3 Rydberg-state molecules from the REMPI experiment. The wavenumber corresponding to the lowest-energy mode (the ring torsion vibration) appears to be 40 cm(-1) in 1 and 35 cm(-1) in the deuterated complexes. The REMPI peaks are homogeneously broadened. The lower lifetime limits for the upper-state components increase on going from the vibrationless level to higher-lying vibronic states and on going from 1 to the deuterated derivatives.

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.

Magnetic field effect on highly excited states near ionization potential of nitric oxide

Fluorescence excitation spectra due to the A 2∑–X 2Π transition of gaseous nitric oxide (NO) were measured in magnetic fields, H; ranging from 0 ≤ H ≤ 10 T. Lines in the observed spectra were assigned by using quantum mechanical calculations. Based on the assignment, two-color resonance-enhanced multiphoton ionization spectra via a single Zeeman sublevel in the A state were measured to observe the effect of magnetic fields on electronic states near the ionization potential (IP). Complicated structures due to strong n- and l-mixing in highly excited Rydberg states were observed below the IP. It was found that new resonance appears above the IP for H ≤ 4T. By using a semi-classical calculation, this resonance was assigned to the quasi-Landau resonance, which was observed for the first time in molecules.© 2003 Elsevier Ltd. All rights reserved.

A REMPI and ZEKE Spectroscopic Study of trans-Acetanilide·H2O and Comparison to Ab Initio CASSCF Calculations

In this paper, two-color REMPI (resonance enhanced multiphoton ionization) and ZEKE (zero kinetic energy photoelectron) spectra of the trans-acetanilide·H2O complex are presented, extending our previous work on the trans-acetanilide monomer (Ullrich, S.; Muller-Dethlefs, K. J. Phys. Chem. A 2002, 106, 9181). Spectral features are interpreted with the aid of ab initio CASSCF/cc-pVDZ calculations and compared to the monomer and related molecular systems, such as trans-formanilide·H2O (Ullrich, S.; et al. Phys. Chem. Chem. Phys. 2002, 4, 2897). In addition, the nature of the excess charge distribution in trans-acetanilide+, and its modification upon solvation are discussed. Complexation of acetanilide with water leads to drastic changes in the S0 and S1 state barrier heights for methyl group internal rotation and possibly reorientation of the methyl group compared to the monomer. Observations are interpreted by simulations of methyl rotor states in a V3−V6 potential.

Microscopic Solvation of Acetonitrile to 9,9‘-Bianthryl Studied in Gas Phase Clusters: Polar Excited State Formation

Dynamics of the electronically excited state of 9,9‘-bianthryl(BA) was studied in gas-phase clusters with acetonitrile (ACN), BA-(ACN)n (n = 1−5). The gas-phase clusters were formed by supersonic expansion of BA/acetonitrile/He mixtures. In the laser-induced fluorescence spectra, new absorption bands ascribed to the BA-(ACN)n clusters were observed at longer wavelength side of the S1−S0 transition of BA. The mass-selected two-color resonance-enhanced multiphoton ionization (REMPI) spectra were measured to determine the absorption band for the cluster of BA with the specific number of the acetonitrile molecules. The dispersed fluorescence (DF) spectra and lifetimes were measured for these absorption bands to study the dynamics of the excited state of BA in the clusters, BA-(ACN)n. For all clusters studied, the DF spectra of BA were broad and largely red shifted compared with that of unclustered BA. The fluorescence lifetimes of BA in the clusters were considerably longer than that of unclustered BA. These ...

Observation of Rotational Isomers I: A ZEKE and Hole-Burning Spectroscopy Study of 3-Methoxyphenol

Three rotational isomers of 3-methoxyphenol have been studied using two-color resonance-enhanced multiphoton ionization (REMPI) and zero kinetic energy (ZEKE) photoelectron spectroscopy. Hole-burning spectroscopy was employed to confirm the presence of three isomers in the REMPI spectrum (S1 band origins were identified at 35 974, 36 034 and 36 202 ± 1 cm-1 for isomers I, IV, and III, respectively). ZEKE spectra recorded via the S1 states of the isomers provide adiabatic ionization energies of 65 228, 64 741, and 65 648 ± 5 cm-1, respectively. The isomers are assigned with the aid of HF/cc-pVDZ, MP2/cc-pVDZ, and B3LYP/cc-pVDZ ab initio and density functional theory calculations. We discuss the characteristics of the 3-methoxyphenol rotational isomers by comparing the REMPI and ZEKE spectra with those of other substituted benzenes.

Real-Time Quantitative Analysis of Combustion-Generated Polycyclic Aromatic Hydrocarbons by Resonance-Enhanced Multiphoton Ionization Time-of-Flight Mass Spectrometry

We have combined resonance-enhanced multiphoton ionization (REMPI) time-of-flight mass spectrometry with on-line flame sampling to determine the centerline concentrations of naphthalene, fluorene, and anthracene in a pure methane + oxygen/argon (1:5) diffusion flame. Naphthalene concentrations between 100 parts per billion by volume (ppbV) and 6 parts per million by volume (ppmV) and fluorene concentrations below 50 ppbV are determined using one-color REMPI on jet-cooled samples extracted from the flame; anthracene concentrations in the 5-40 ppbV range are determined using two-color REMPI. The REMPI ion signals are converted to absolute concentrations in real time by performing gas-phase standard additions to the flame sample. Isomer-selective detection of larger polycyclic aromatic hydrocarbons, such as perylene and benzo[a]pyrene, is possible using the two-color REMPI approach.

Experimental and theoretical characterization of the S1–S transition of benzo[a]pyrene

We use fluorescence excitation and two-color resonance-enhanced multiphoton ionization (REMPI) spectroscopy to characterize the S1–S0 transition of benzo[a]pyrene, B[a]P. In addition, two-color REMPI with a tunable ionization laser provides an improved measurement of the molecular ionization potential, 7.103±0.004 eV. Ab initio calculations are performed to determine the vibrational frequencies for the 90 vibrational modes in the S1 and S0 electronic states. Our results indicate that the single-excitation configuration interaction (CIS) level of theory provides an excellent representation of excited-state frequencies for this large molecule. Virtually all of the vibronic bands between ∼380 nm and 397 nm, as well as numerous bands between 373 nm and 380 nm, are assigned with the aid of the calculated vibrational frequencies. In total, 27 of the 61 a′ symmetry vibrational modes and 5 of the 29 a″ vibrational modes in the S1 state have now been assigned.

Photoionization dynamics of the NO A 2Σ+ state deduced from energy- and angle-resolved photoelectron spectroscopy

The direct photoionization process NO A 2Σ+ (v=0, N=13)→NO+ X 1Σ+ (v+=0, N+)+e− is studied by energy- and angle-resolved photoelectron spectroscopy by employing two-color resonance-enhanced multiphoton ionization (REMPI) via excitation of the NO A–X(0–0) R21(11.5) transition. The photoelectron angular distributions (PADs) associated with individual rotational levels N+ of the ion are determined. Combined analysis of the newly obtained PADs and those reported earlier for the processes NO A 2Σ+ (v=0, N≥20)→NO+ X 1Σ+ (v+=0, N+)+e− via P21+Q1 branch excitation shows that the photoionization dynamics is independent of the rotational quantum number of state to be ionized and of the spin state of the photoelectron. Quantitative comparison of our results with threshold photoelectron measurements provides strong evidence that ionization in the pulsed-electric-field threshold technique is not via direct photoionization.

The lowest excited singlet state of isolated 1-phenyl-1,3-butadiene and 1-phenyl-1,3,5-hexatriene

We report vibrationally resolved S0→S1 excitation spectra and vibronic level decay times for the phenyl-substituted polyenes 1-phenylbutadiene and 1-phenylhexatriene seeded in supersonic He expansions. This information was obtained using one- and two-color resonance-enhanced multiphoton ionization techniques. The shift in the excitation energy of the lowest excited singlet state upon deuteration of the phenyl ring demonstrates that in 1-phenylbutadiene S1 is mainly an excitation of the benzene ring while in 1-phenylhexatriene S1 is mainly the 2 1Ag state of hexatriene. Analysis of the excitation spectrum of 1-phenylhexatriene shows that the spectrum contains contributions from two species, the trans,trans and the trans,cis isomers, whose 2 1Ag state excitation energies differ by about 155 cm−1. The lifetimes of 2 1Ag zero-point levels and, in particular, the activity of the C–C and C=C stretch vibrations are markedly different for the two isomers. From these results we obtain a picture of the influence that the phenyl group has on such aspects of the electronic structure of the 2 1Ag state of hexatriene as the effective conjugation length, the symmetry properties, and the fluorescence quantum yield.

Electronic spectra of p-difluorobenzene cation by mass-selected ion dip spectroscopy in a supersonic jet

The 2B 3u(π, π)←D 0( 2B 2g) transition of p-difluorobenzene ( p-DFB) cations prepared by two-color resonance-enhanced multiphoton ionization (REMPI) has been observed by mass-selected ion dip spectroscopy utilizing dissociation. All the spectra due to the transitions from different vibrational levels in D 0 showed vibrationally resolved structures. It was found from their vibrational analysis that there exists a strong vibronic coupling between the 2B 3u(π, π) state and the nearby (σ, π) state, and the former is the lowest excited state. The nonemissive property of p-DFB cation forms an exception to the generally accepted criterion that all fluorobenzene cations having the lowest excited (π, π) state are emissive.