Two-color Resonant Two-photon Mass-analyzed Threshold Research Articles

Two-Color Resonant Two-Photon Mass-Analyzed Threshold Ionization of 2,4-Difluoroanisole and the Additivity Relation of Ionization Energy.

We used the two-color resonant two-photon ionization and mass-analyzed threshold ionization spectroscopic techniques to record the vibronic, photoionization efficiency, and cation spectra of 2,4-difluoroanisole. The cation spectra were obtained through ionization via seven intermediate vibronic states, which involved out-of-plane ring-F, in-plane ring-F, and ring-OCH3 bending vibrations as well as in-plane ring deformation vibrations. The band origin of the S1 ← S0 electronic transition of 2,4-difluoroanisole appeared at 35 556 ± 2 cm-1, and the adiabatic ionization energy was determined to be 67 568 ± 5 cm-1. The experimental data provided information on the active vibrations of aforementioned compound in the electronically excited state S1 and ground cationic state D0. A comparison of the experimental data obtained for 2,4-difluoroanisole in this study with the data obtained for other fluorine-substituted benzenes in previous studies indicated that the nature, location, and number of substituents influence electronic transition energy, ionization energy, and molecular vibration. In addition, a simple additivity relation might exist for predicting the ionization energy of multiply substituted benzene derivatives.

Cation spectra of p-chloroanisole and the heavy atom effect on ionization energy

Vibrationally resolved cation spectra of p-chloroanisole were recorded by ionizing via four vibronic levels through two-color resonant two-photon mass-analyzed threshold ionization (MATI) spectroscopy techniques. The results indicated that the geometry and molecular vibrations 7a, 16b, and 6a of p-chloroanisole in the cationic ground D0 state resemble those in the electronically excited neutral S1 state. The precisely measured adiabatic ionization energy of this molecule was less than that of p-fluoroanisole by 325 cm−1. This can be labeled as a heavy atom effect, which is similar to that observed for para halogen-substituted phenol and aniline.

Cation Vibrations of 1-Methylnaphthalene and 2-Methylnaphthalene through Mass-Analyzed Threshold Ionization Spectroscopy.

Vibrationally resolved cation spectra of 1-methylnaphthalene (1MN) and 2-methylnaphthalene (2MN) were obtained using the two-color resonant two-photon mass-analyzed threshold ionization (MATI) spectroscopy. MATI spectra were obtained through ionization via several intermediate vibronic levels. Due to hindrance effect, no spectral features related to methyl torsion were observed in the MATI spectra of 1MN. By contrast, most of the in-plane ring deformation vibrations of the 2MN cation were found to couple with methyl torsion because of its small internal rotational barrier. These experimental findings were well supported by our theoretical calculations.

Ionization energy and active cation vibrations of trans-2-fluorostyrene

We applied the two-color resonant two-photon mass-analyzed threshold ionization (MATI) technique to record the cation spectra of trans-2-fluorostyrene by ionizing via six intermediate vibronic levels. The adiabatic ionization energy was determined to be 69304±5cm−1. The distinct MATI bands at 67, 124, 242, 355, 737, 806, 833, and 993cm−1 were assigned to the active cation vibrations related to out-of-plane substituent-sensitive bending vibrations and in-plane ring deformation and bending motions. Many combination vibrations were also observed. Our experimental results suggest that the molecular geometry and vibrational coordinates of the trans-2-fluorostyrene cation in the D0 state resemble those of the neutral species in the S1 state.

Selected cis- and trans-3-fluorostyrene rotamers studied by two-color resonant two-photon mass-analyzed threshold ionization spectroscopy

We applied two-color resonant two-photon ionization and mass-analyzed threshold ionization techniques to record the vibronic, photoionization efficiency, and cation spectra of the selected rotamers of 3-fluorostyrene. The adiabatic ionization energies of cis- and trans-3-fluorostyrene were determined to be 69 960±5 and 69 856±5cm−1, respectively. Cation vibrations 10a, 15, 6b, and 12 of both rotamers have been found to have frequencies of 218, 404, 452, and 971cm−1, respectively. This finding shows that the relative orientation of the vinyl group with respect to the F atom does not affect these vibrations of the 3-fluorostyrene cation. Our one-dimensional potential energy surface calculations support that the cis–trans isomerization of 3-fluorostyrene does not occur under the present experimental conditions.

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.

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.

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.

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.

Cation spectroscopy of o-fluoroanisole and p-fluoroanisole by two-color resonant two-photon mass-analyzed threshold ionization

We applied the two-color resonant two-photon mass-analyzed threshold ionization spectroscopic technique to record the cation spectra of o-fluoroanisole and p-fluoroanisole. The adiabatic ionization energies of these two structural isomers were found to be 67354±5 and 66437±5cm−1, respectively. The observed active normal modes of the cations mainly result from the in-plane ring and substituent-sensitive vibrations. The results suggest that the relative location of the F and OCH3 substituents can slightly influence the transition energy and molecular vibrations.

Cation spectroscopy of 3,4-difluoroaniline by two-color resonant two-photon mass-analyzed threshold ionization

We applied the two-color resonant two-photon mass-analyzed threshold ionization technique to record the vibrationally resolved cation spectra of 3,4-difluoroaniline (34DFA) via the 0 0, X 1 , 6b 1, and I 2 levels of the S 1 state. The adiabatic ionization energy of this molecule was determined to be 64 195 ± 5 cm −1. Most of the observed active modes of the 34DFA cation in the D 0 state are related to the in-plane ring deformation vibrations. Comparing these data with those of 3-fluoroaniline and 4-fluoroaniline, one can learn the effects of fluorine substitution on the electronic transition and molecular vibration.

Rotamers of o- and m-Dimethoxybenzenes Studied by Mass-Analyzed Threshold Ionization Spectroscopy and Theoretical Calculations

We applied two-color resonant two-photon mass-analyzed threshold ionization (MATI) spectroscopy to investigate the molecular properties of the selected rotamers of o-dimethoxybenzene (ODMB) and m-dimethoxybenzene (MDMB). The present experimental results show that only one stable configuration is involved in the photoexcitation and ionization processes of ODMB, as predicted by theoretical calculations. The adiabatic ionization energy (IE) of ODMB is measured to be 61 617 ± 5 cm(-1). In the case of MDMB, both our experimental and calculated results suggest that there are three stable rotamers coexisting in the sample. The adiabatic IEs of rotamers a, b, and c are determined to be 63 523 ± 5, 64 491 ± 5, and 63 758 ± 5 cm(-1). Analysis on the MATI spectra shows that most of the active cation vibrations of these isomeric species result from in-plane ring motions. In addition, different orientations of the two OCH(3) groups have little effect on these vibrations.

Mass-analyzed Threshold Ionization Spectroscopy of Rotamers ofp-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 cm1, which are less than that of p-methoxyphenol by 645 and 643 cm1, 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.

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.

Mass-analyzed threshold ionization spectroscopy of the rotamers of p-n-propylphenol 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-n-propylphenol. The adiabatic ionization energies of the trans, gauche-A, and gauche-B rotamers are determined to be 65 283+/-5, 65 385+/-5, and 65 369+/-5 cm(-1), which are less than that of phenol by 3342, 3240, and 3256 cm(-1), respectively. This suggests that the n-propyl substitution causes a greater degree in lowering the energy level in the cationic than the neutral ground state. Analysis on the MATI spectra of the selected rotamers of p-n-propylphenol cation shows that the relative orientation of the p-n-alkyl group has little effect on the in-plane ring vibrations. However, the low-frequency C(3)H(7) bending vibrations appear to be active only for the two gauche forms of the cation.

Identification of Impurities in Phenylacetylene by Species-Selected Mass-Analyzed Threshold Ionization Spectroscopy

Two-color resonant two-photon mass-analyzed threshold ionization (MATI) spectroscopy was used to identify styrene impurities in a commercial phenylacetylene sample. With the mass-gating setup, the recorded MATI spectra are free of contamination from molecules of different mass. Analysis of these spectra gives information on the precise adiabatic ionization and the active cation vibrations, which can be used as fingerprints for the identification of molecular ions.