Mass-analyzed Threshold Ionization Spectroscopy of Rotamers of p-ethoxyphenol Cations and Configuration Effect

Two-color resonant two-photon mass-analyzed threshold ionization （MATI） spectroscopy was used to record the vibrationally resolved cation spectra of the selected rotamers of p- ethoxyphenol. The adiabatic ionization energies of the trans and cis rotamers are determined to be 61565±5 and 61670±5 cm^-1, which are less than that of p-methoxyphenol by 645 and 643 cm^-1, respectively. Analysis on the MATI spectra of the selected rotamers of p-ethoxyphenol cation shows that the relative orientation of the ethoxy group has little effect on the in-plane ring vibrations. The low-frequency OC2H5 bending vibrations appear to be active for both forms of the cation.     

Mass-analyzed threshold ionization spectroscopy of p-methylphenol and p-ethylphenol cations and the alkyl substitution effect

The mass-analyzed threshold ionization (MATI) spectra of p-methylphenol and p-ethylphenol have been recorded by ionizing via various vibronic levels. The adiabatic ionization energies (IEs) of p-methylphenol and p-ethylphenol are determined to be 65918+/-5 and 65628+/-5 cm(-1), which are less than that of phenol by 2707 and 2997 cm(-1), respectively. This redshift indicates that the interaction between the alkyl group and the ring of alkylphenols in the cationic D(0) state is greater than that in the neutral S(0) state. Moreover, a longer alkyl group gives rise to a greater redshift in the IE. Analysis of the MATI spectra shows that most of the active modes are related to the in-plane ring vibrations of these two cations. However, the length of the alkyl group has an insignificant effect on the frequency of the observed ring vibrations. No band with frequency less than 350 cm(-1) is observed for the p-methylphenol cation. In contrast, many low-frequency bands resulting from the characteristic motions (e.g., the C-C(2)H(5) torsion and C-C(2)H(5) and C-OH bending vibrations) appear in the MATI spectra of p-ethylphenol. The present results show that the ethyl group enhances the substituent-sensitive and many large-amplitude vibrations of the cation.     

Rotamers of m-cresol cation studied by mass-analyzed threshold ionization spectroscopy

We have applied two-color resonant two-photon mass-analyzed threshold ionization technique to record the vibrational spectra of the selected rotamers of m-cresol. The adiabatic ionization energies of cis and trans m-cresol are determined to be 66,933+/-5 and 67,084+/-5 cm(-1), respectively. Frequencies of the in-plane ring vibrations 6a, 1, and 9b are measured to be 528, 720, 1167 cm(-1) for the cis and 520, 698, and 1153 cm(-1) for the trans m-cresol cation. This indicates that different orientation of the OH group with respect to the CH(3) group slightly influences these ring vibrations.     

Vibrations and theoretical calculations of p-methylanisole in the first electronically excited S1 and ionic ground D0 states

One-color (1C), two-color (2C) resonant two-photon ionization (R2PI), and mass analyzed threshold ionization (MATI) methods have been applied to study the S(1)<--S(0) transition and threshold ionization of p-methylanisole. The excitation energy of the S(1)<--S(0) transition is determined to be 35,401+/-2 cm(-1), the adiabatic ionization energy of this molecule is measured to be 63,965+/-15 and 63,972+/-5 cm(-1) by the 2C-R2PI and MATI methods. Most of the observed R2PI and MATI bands result from the in-plane ring vibrations. The frequencies of vibrations 9b, 1 and 7a are measured to be 393, 800 and 1168 cm(-1) in the S(1) state, and 412, 811 and 1220 cm(-1) in the D(0) state, respectively. This indicates the molecular structure in the D(0) state is more rigid than that in the S(1) state.     

One-color two-photon mass-analyzed threshold ionization spectroscopy of ethyl bromide through a dissociative intermediate state

Mass-analyzed threshold ionization (MATI) spectra of ethyl bromide were obtained using one-color two-photon ionization through a dissociative intermediate state. Accurate values for the adiabatic ionization energy have been obtained, 83099+/-5 and 85454+/-5 cm(-1) for the X1 2E and X2 2E states of the ethyl bromide cation, respectively, giving a splitting of 2355+/-10 cm(-1). Compared with conventional photoelectron data, the two-photon MATI spectrum exhibited a more extensive vibrational structure with a higher resolution, mainly containing the modes involving the dissociation coordinate. The observed modes were analyzed and discussed in terms of wave packet evolving on the potential-energy surface of the dissociative state.     

2, 5-二氟苯酚分子之旋转异构物的质量分辨阈值电离光谱

利用共振双光子电离(R²PI)技术和质量分辨阈值电离(MATI)技术来研究2, 5-二氟苯酚分子。实验所测得的顺式、反式2, 5-二氟苯酚分子电子激发能E₁分别为36448和36743 cm^-1,绝热电离能分别为71164和71476 cm^-1。这两个顺反转动同素异构分子在电子激发S₁态与离子D₀态活性振动主要是由于面内环变形和与取代基相关的弯曲振动。分析2, 5-二氟苯酚分子的振动光谱、D₀态离子光谱以及理论计算均表明这两个转动异构体在D₀态的几何构型与S₁态的中性几何构型相当相似。     

Mass-analyzed threshold ionization spectroscopy of pyridine. Structural distortion in the first excited state

For the first time, vibrational spectra of the pyridine cation in the electronic ground state have been measured via several intermediate states (0(0), 16b0(2), 16b0(4), 6a0(1), 6b(1), 16a0(1), 10a0(1) and 12(1)) by Mass-Analyzed Threshold Ionization (MATI) spectroscopy. From the MATI spectra, the adiabatic ionization energy of pyridine has been determined to be 74,185 +/- 6 cm(-1) (9.1978 +/- 0.0008 eV). Several vibronic modes in the ionic ground state could be assigned for the first time. An intensity gain of vibrations having b1 symmetry could be observed by activating the ion ground state. Also, a breakdown of the "delta nu = 0 propensity rule" for the excitation via the 16b(2) and 16b(4) states of the first excited states are displayed in the recorded spectra. In conjunction with ab initio calculations these observations can be explained by a strong geometrical distortion along the 16b vibration in the first excited state, leading to a "boat distortion".     

Mass analyzed threshold ionization spectroscopy of 5-methylindole and 3-methylindole cations and the methyl substitution effect

The vibrationally resolved mass analyzed threshold ionization spectra of jetcooled 5-methylindole (5MI) and 3-methylindole (3MI) have been recorded by ionizing via various vibronic levels of each species. The adiabatic ionization energies (IEs) of 5MI and 3MI are determined to be 61,696+/-5 and 60,679+/-5 cm(-1), which are less than that of indole by 895 and 1912 cm(-1), respectively. Comparing these data with those of 1-methylindole and indole suggests that the methyl substitution on the pyrrole part leads to a greater redshift in the IE than on the benzene part. These experimental findings are well supported by the theoretical calculations. Analysis on these new data shows that many active vibrations of the 5MI cation are related to the CH(3) torsion and in-plane ring bending vibrations. In contrast, the observed vibrational bands of the 3MI cation are very weak due to unfavorable Franck-Condon transition.     

Mass analyzed threshold ionization spectroscopy of o-, m-, and p-dichlorobenzenes. Influence of the chlorine position on vibrational spectra and ionization energy

For the first time, vibrational spectra of the 35Cl2 and 35Cl37Cl isotopomers of o-, m-, and p-dichlorobenzene cations in the electronic ground state have been measured via S1 intermediate states by mass analyzed threshold ionization (MATI) spectroscopy. Additionally, ab initio calculations at DFT (density functional theory), CIS (configuration interaction singles), and CASSCF (complete active space self-consistent field) levels of theory have been conducted to compare experimental findings with theory. From the MATI spectra, adiabatic ionization energies of the ortho, meta, and para isomers have been determined to be the same for each pair of investigated isotopomers to 73,237 +/- 6, 72,191 +/- 6, and 73,776 +/- 6 cm(-1), respectively. Several vibrational modes, including fundamentals, combinations, and progressions have been assigned by comparing the experimental and theoretical results. The appearance of overtone progressions involving the 7a mode could be explained by a geometry change of all three isomers during ionization in the direction of this mode by retraining the symmetry of the molecules. Although the general spectral features of the investigated isotopomers are similar, frequencies of some vibrations are slightly different up to a few wavenumbers depending on the involvement of the chlorine atoms in the molecular motion.     

Mass analyzed threshold ionization spectroscopy of 7-azaindole cation

The vibrationally resolved mass analyzed threshold ionization (MATI) spectra of jet-cooled 7-azaindole have been recorded by ionizing via four different intermediate levels. The adiabatic ionization energy of this molecule is determined to be 65 462±5 cm⁻¹, which is greater than that of indole by 2871 cm⁻¹. The vibrational spectra of 7-azaindole in the S₁ and D₀ states have been successfully assigned by comparing the measured frequencies with those of indole as well as the predicted values from the ab initio calculations. Detailed analysis on the MATI spectra shows that the structure of the cation is somewhat different from that of this species in the neutral S₁ state.     

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.     

Photodissociation yield spectroscopy of vinyl bromide cation generated by mass-analyzed threshold ionization: vibrational spectroscopy and decay dynamics in the (~)B state

A new technique [mass-analyzed threshold ionization (MATI)-photodissociation yield spectroscopy] to probe bound excited states of a cation was developed, which measures photodissociation yield of the cation generated by mass-analyzed threshold ionization. A vibrational spectrum of vinyl bromide cation in the (~)B state was obtained using this technique. Optical resolution in the low vibrational energy range of the spectrum was far better than in conventional MATI spectra. The origin of the (~)B state was found at 2.2578+/-0.0003 eV above the first ionization onset. Almost complete vibrational assignment was possible for peaks appearing in the spectrum. Analysis of time-of-flight profiles of C(2)H(3) (+) product ion obtained with different laser polarization angles suggested that photoexcited vinyl bromide cation remained in the (~)B state for several hundred picoseconds prior to internal conversion to the ground state and dissociation therein.     

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.     

Mass-analyzed threshold ionization study of vinyl bromide cation in the first excited electronic state using vacuum-ultraviolet radiation generated by four-wave mixing in Hg

The vibrational spectrum of the vinyl bromide cation in the first excited electronic state A 2A' was obtained by one-photon mass-analyzed threshold ionization (MATI) spectroscopy. The use of an improved vacuum-ultraviolet radiation source based on four-wave sum frequency mixing in Hg resulted in excellent sensitivity for MATI signals. From the MATI spectrum, the ionization energy to the A 2A' state of the cation was determined to be 10.9150+/-0.0006 eV. Nearly complete vibrational assignments for the MATI peaks were possible by utilizing the vibrational frequencies and Franck-Condon factors calculated at the density-functional theory (DFT) and time-dependent DFT/B3LYP levels with the 6-311+G(df,p) basis set.     

间甲基苯甲醚分子旋转异构体的质量分辨阈值电离光谱

间甲基苯甲醚分子有顺式和反式两个转动异构体.利用单光共振双光子电离技术和质量分辨阈值电离技术,研究了间甲基苯甲醚分子顺反异构体的基态到第一电子激发态(S1←S0)的跃迁和阈值电离.得到顺式、反式间甲基苯甲醚分子S1态的激发能(E1)分别为(36049±2)和(36117±2)cm-1,绝热电离能(Ip)分别为(64859±5)和(65110±5)cm-1.结合从头算法和密度泛函理论的量子化学计算,解释了顺式、反式间甲基苯甲醚分子E1和Ip存在差异的原因,并且对S1态和离子基态D0态出现的谱峰进行了标识.间甲基苯甲醚分子顺反异构体在S1态和D0态的活性振动主要是甲基转动、面内环的运动和与取代基相关的弯曲振动.间甲基苯甲醚分子的S1态振动光谱、D0态离子光谱以及理论计算均表明这两个转动异构体在D0态的几何构型与S1态的中性几何构型相比有较大改变,取代基与取代基、取代基与苯环间的相互作用强度高低次序为:S0S1D0.     

Mass analyzed threshold ionization spectra of phenol...Ar2: ionization energy and cation intermolecular vibrational frequencies

The phenol(+)...Ar(2) complex has been characterized in a supersonic jet by mass analyzed threshold ionization (MATI) spectroscopy via different intermediate intermolecular vibrational states of the first electronically excited state (S(1)). From the spectra recorded via the S(1)0(0) origin and the S(1)β(x) intermolecular vibrational state, the ionization energy (IE) has been determined as 68,288 ± 5 cm(-1), displaying a red shift of 340 cm(-1) from the IE of the phenol(+) monomer. Well-resolved, nearly harmonic vibrational progressions with a fundamental frequency of 10 cm(-1) have been observed in the ion ground state (D(0)) and assigned to the symmetric van der Waals (vdW) bending mode, β(x), along the x axis containing the C-O bond. MATI spectra recorded via the S(1) state involving other higher-lying intermolecular vibrational states (σ(s)(1), β(x)(3), σ(s)(1)β(x)(1), σ(s)(1)β(x)(2)) are characterized by unresolved broad structures.     

Mass-analyzed threshold ionization study of vinyl chloride cation in the first excited electronic state using vacuum ultraviolet radiation in the 107-102.8 nm range

Vibrational spectrum of vinyl chloride cation in the first excited electronic state, A2 A', was obtained by one-photon mass-analyzed threshold ionization (MATI) spectroscopy. Use of an improved vacuum ultraviolet radiation source based on four-wave sum frequency mixing in Hg resulted in excellent sensitivity for the MATI signals. From the MATI spectrum, the ionization energy to the A2 A' state of the cation was determined to be 11.6667 +/- 0.0006 eV. Nearly complete vibrational assignment for the MATI peaks was possible by utilizing the vibrational frequencies and Franck-Condon factors calculated at the DFT and TDDFT/B3LYP levels with the 6-311++G(3df,3pd) basis set. Geometry of the cation in the A2 A' state was determined by Franck-Condon fitting of the MATI spectrum.     

Ionization spectroscopy of conformational isomers of propanal: the origin of the conformational preference

Two different conformational isomers of propanal, cis and gauche, are investigated by the vacuum-UV mass-analyzed threshold ionization (VUV-MATI) spectroscopy to give accurate adiabatic ionization potentials of 9.9997 +/- 0.0006 eV and 9.9516 +/- 0.0006 eV, respectively. cis-Propanal, which is the more stable conformer in the neutral state, becomes less stable in the cation compared to gauche-propanal. Vibrational structures revealed in the MATI spectra indicate that cis and gauche isomers undergo their unique structural changes upon ionization. The ionization of gauche-propanal induces a geometrical change along the conformational coordinate, suggesting that the steric effect in the ground state is diminished upon ionization. Natural bonding orbital (NBO) calculations provide the extent of hyperconjugation in each conformational isomer of propanal.     

间甲基苯甲醚分子旋转异构体的质量分辨阈值电离光谱

间甲基苯甲醚分子有顺式和反式两个转动异构体. 利用单光共振双光子电离技术和质量分辨阈值电离技术,研究了间甲基苯甲醚分子顺反异构体的基态到第一电子激发态（S₁←S₀）的跃迁和阈值电离. 得到顺式、反式间甲基苯甲醚分子S₁态的激发能（E₁）分别为（36049±2）和（36117±2）cm^-1,绝热电离能（I_p）分别为（64859±5）和（65110±5）cm^-1. 结合从头算法和密度泛函理论的量子化学计算,解释了顺式、反式间甲基苯甲醚分子E₁和I_p存在差异的原因,并且对S₁态和离子基态D0态出现的谱峰进行了标识. 间甲基苯甲醚分子顺反异构体在S₁态和D₀态的活性振动主要是甲基转动、面内环的运动和与取代基相关的弯曲振动. 间甲基苯甲醚分子的S₁态振动光谱、D₀态离子光谱以及理论计算均表明这两个转动异构体在D₀态的几何构型与S₁态的中性几何构型相比有较大改变,取代基与取代基、取代基与苯环间的相互作用强度高低次序为：S₀<S₁<D₀.     

Spectral shifts and structures of phenol...Ar(n) clusters

A laser spectroscopic investigation of phenol...Ar(n) (n = 1-6) clusters in the first electronically excited state (S(1)) and the cationic ground state (D(0)) is reported. Resonance enhanced two-photon ionisation (R2PI) spectra have been recorded for the investigation of the S(1) state. The origins of S(1)← S(0) (S(1)0(0)) transition of phenol...Ar(n) (n = 1, 2,4-6) are all red shifted compared to the S(1)0(0) state of the monomer by 33 cm(-1), 67 cm(-1), 10 cm(-1), 20 cm(-1), 44 cm(-1), respectively. However, the origin of the phenolAr(3) cluster is blue shifted by 25 cm(-1). For the investigation of the ionic ground state photoionization efficiency (PIE) and mass-analyzed-threshold ionization (MATI) spectroscopy have been applied. The spectra of phenol...Ar(3) and phenol...Ar(4) yield values for the ionization energy (IE) of 68,077 ± 15 cm(-1) and 67,948 ± 15 cm(-1). With the combination of theoretical methods and R2PI, PIE and MATI spectroscopy, the major species present have been positively identified.