Chemical Physics Letters 380 (2003) 337–341

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Vibrational levels of p-xylene cation determined bymass-analyzed threshold ionization spectroscopy

Bing Zhang a,*, Udo Aigner b, Heinrich Ludwig Selzle b, Edward William Schlag b

a State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics,

Chinese Academy of Sciences, Wuhan 430071, PR Chinab Institute fuer Physikalische und Theoretische Chemie der Technischen Universitaet Muenchen, Lichtenbergstrasse 4,

Garching D-85748, Germany

Received 27 June 2003; in final form 5 September 2003

Published online: 28 September 2003

Abstract

Mass-analyzed threshold ionization (MATI) spectroscopy and two-color resonant two-photon ionization method

were used for the determination of the vibrational levels of the p-xylene cation. The MATI spectrum was recorded via

the 00 vibrationless level of the S1 state of p-xylene. The spectrum shows a rich structure and some vibrational fre-

quencies of the cation are determined. The experimental findings are well supported by ab initio calculation.

� 2003 Elsevier B.V. All rights reserved.

1. Introduction

ZEKE spectroscopy [1–3] has emerged now as a

new high-resolution spectroscopy of molecular

ions. The spectroscopy in this high-resolution form

depends on the formation of long-lived Rydberg

states. It can be used to determine the ionization

energy (IE) of molecules as well as vibrational

levels of ions in high precision. Since the ZEKE

technique is subject to the detection of electrons, ithas no mass information unless coupled with mass

coincidence. In a similar approach, mass-analyzed

threshold ionization (MATI) method involves de-

tection of ions rather than electrons and thus can

* Corresponding author. Fax: +862787199291.

E-mail address: bzhang@wipm.ac.cn (B. Zhang).

0009-2614/$ - see front matter � 2003 Elsevier B.V. All rights reserv

doi:10.1016/j.cplett.2003.09.033

provide an unambiguous mass resolved spectral

information [4–8].Benzene and its derivative are of great interest

in biological and material science as well as in

pharmaceutical and chemical industries. The nor-

mal vibrations of p-xylene in ground state have

been characterized on the basis of IR, far IR, and

Raman spectroscopic data [9]. The electronic

transition was investigated by absorption spec-

trum of the solid phase [10] and ð1þ 1Þ-photo-ionization spectra [11,12]. The spectrum of the

transition to the first electronic excited state of

p-xylene with one color REMPI [13,14] has been

studied previously in detail [15] and the vibronic

structure was assigned.

The p-xylene ions were investigated by He(I)

photoelectron (PE) spectroscopy [16] and mul-

tiphoton ionization and means of time of flight

ed.

338 B. Zhang et al. / Chemical Physics Letters 380 (2003) 337–341

photoelectron (TOFPE) spectroscopy [15]. A few

vibrational energies of the ionic ground state were

reported. However, the assignments of the spectra

were rough in these papers and the vibrational

energies of the ionic ground state were imprecise

due to the poor resolution of the PE spectroscopy.A MATI spectrum over a range of 550 cm�1 has

been reported [17] with a detailed study of the

internal rotation of the methyl groups of the

p-xylene cation.

In this work, we present a spectrum of the

p-xylene ion over a extended region of 1500 cm�1

and determine the ionic vibrational levels of the

p-xylene cation with high precision. We have alsoconducted ab initio calculation to predict the

structure, energies and vibrational energies of

p-xylene in the S0, S1 and the ionic ground state.

The results from the calculations are found to

support well our experimental findings.

2. Experimental

The experiment reported in this paper was

performed with a laser-based TOF mass spec-

trometer as described in previous publication [18].

Briefly, p-xylene at 10 �C seeded in Ar at 4 bar is

expanded through a pulsed nozzle with a 0.2 mm

orifice into the vacuum. The supersonic jet is

skimmed 5 cm downstream and then again by asecond skimmer mounted on the first plate of the

ion optics. The molecules, after passing through

the skimmers, interact with two counterpropagat-

ing pulsed lasers. The first laser is tuned to excite

the resonant S1 00 intermediate state. Transition

from this state through the Rydberg manifold are

accessed by scanning the second frequency dou-

bled laser in the range from 31 268 to 32 868 cm�1.The laser intensities were adjusted so that the di-

rect ions produced through one-color absorption

of each laser are negligible compared to those

produced through two-color absorption. The ab-

solute frequencies of the lasers are determined

using a high resolution wavemeter. The relative

frequencies are calibrated by comparison with a

simultaneously recorded iodine spectrum.In the MATI experiment, both the prompt ions

and the Rydberg neutral are formed simulta-

neously in the laser/molecular beam interaction

zone between the first two plates of the ion optics.

The prompt ions and the excited neutral molecules

then drift with the speed of the molecular beam

into the region between the second and third plate.

About 100 ns after the occurrence of the laserpulses, a pulsed electric field (spoiling field)

of )100 mV/cm was switched on to reject the

prompt ions from entering the extraction region.

After time delay of about 25 ls, a second pulsed

electric field of +100 V/cm was applied to ionize

Rydberg neutrals which then are extracted into the

reflection time-of-flight (RETOF) mass spectrom-

eter. MATI spectrum is obtained by putting adetection gate at the mass of p-xylene on the TOF

signal. If no spoiling field was applied also the

direct ions could be detected and ionization effi-

ciency (IE) spectra could be obtained.

Ab initio and DFT calculations were performed

using the GAUSSIANAUSSIAN 98 [19] program package.

3. Results and discussion

When the first exciting laser wavelength is tuned

to the 0–0 S1 S0 absorption band of p-xyleneand the wavelength of the second ionization laser

was set to 31 746 cm�1, i.e., the sum of energy of

two laser is just a little above the ionization po-

tential of 8.4537 eV of p-xylene, one can observetwo mass peaks in the TOF spectrum at 73.1 and

73.5 ls (Fig. 1), which correspond to mass 106

and 107. Here, the spoiling field was not applied

and the signal from the direct ions was recorded.

The small mass peak at 107 amu corresponds to13C-labeled p-xylene, where one 13C atom replaces

a carbon of p-xylene. The natural abundance of13C is 1.108%. In p-xylene, however, the naturalabundance of 13C-labeled molecules amounts to

more than 8% due the presence of eight carbon

atoms in molecule. The observed ratio of intensi-

ties of two peaks in Fig. 1 is in agreement with the

natural abundance. This allows to obtain vibra-

tionally resolved REMPI spectra of the first elec-

tronic S1 state of p-xylene and 13C-labeled p-xylenesimultaneously by scanning the wavelength of theresonant exciting dye laser and detection of the

ions at two fixed masses of 106 and 107 amu.

Fig. 1. TOF mass spectrum of ð1þ 1Þ two-photon ionization of

p-xylene from a mixture with natural abundance of 13C.

B. Zhang et al. / Chemical Physics Letters 380 (2003) 337–341 339

The vibrational structure and the 13C isotope effect

in the excitation spectra of p-xylene have been

studied in our previous work [20].

When the exciting laser wavelength is tuned to

the 0–0 S0 S1 absorption band of p-xylene and

no spoiling field was applied in the ionization re-

gion, one obtains the 2C-R2PI IE spectrum ofp-xylene by scanning the ionization laser (Fig. 2).

Fig. 2. Field free RC-R2PI photoionization efficiency spectrum

of p-xylene. The long onset belongs to field ionization of long-

lived Rydberg states.

The spectrum shows a fast rising step at two-pho-

ton energy near 68 186 cm�1, corresponding to the

ionization threshold of p-xylene. Here, the ioniza-

tion threshold can be obtained from the upper edge

of the IE step. Due to the detection scheme with

pulsed extraction of the ions with an electric field of100 V/cm also the still present long-lived neutrals

below the ionization limit are field ionized and

contribute to the ion current. The explains the ex-

tension of the onset of 15 cm�1 to the red and is the

contribution of the long-lived Rydberg states just

below the ionization potential [21].

The mass-analyzed threshold ionization spec-

trum of p-xylene recorded via the vibrationlesslevel 00 (36 732 cm�1) of the S1 state is obtained

(Fig. 3) when a pulsed spoiling field and delayed

pulsed ionization field were applied. The x-axis in

Fig. 3 is total excitation energy, which is sum of

photon energy of pumping laser and ionization

laser. The strongest band appears at two-photon

energy of 68 188 cm�1 and corresponds to the

production of vibrationless ions. The width of thisband is about 12 cm�1 (FWHM), which is due to

the long-lived Rydberg states below the IP and

some convolution of the molecular rotations ex-

cited in the intermediate state [22]. Since MATI

spectroscopy involves ionization of molecules in

the high Rydberg state by a delayed pulsed electric

field, the corresponding ion signal occurs at energy

slightly below the ionization threshold. The de-termination of the ionization threshold should

consider the value in the high-energy side of the 00

band of the molecular ion [23]. The field-corrected

adiabatic IP is found to be 68 186(2) cm�1. This

value is in very good agreement with that mea-

sured by the two-color R2PI spectroscopy (Fig. 2).

The weak features to higher energy of origin are

due to methyl torsions, which have been studied in[17]. The p-xylene possesses 48 normal vibrations,

which include 30 benzene-like and 18 methyl

modes. Here, we only discuss the vibrations, which

are related to the observed bands in the MATI

spectrum. The assignment was made on the basis

of our ab initio calculations and conformity with

the available data in the S0 and S1 states. Calcu-

lated results show that the p-xylene cation has aC2h symmetry. The intense band shifted from the

00 band by 440 cm�1 is assigned to t9b, which is the

Fig. 3. Mass-analyzed threshold ionization spectrum of p-xylene.

Table 1

Peak positions of assigned transitions in the MATI spectrum

Frequency (cm�1) Assignment

68 188 00068 529 31068 628 9b1

0

68 659 6a1068 869 32068 989 6b1

0

69 065 9b20

69 135 6a2069 178 11069 191 11069 219 33069 375 9a1069 435 7a1069 478 9b3

0

69 581 34069 632 6a30

340 B. Zhang et al. / Chemical Physics Letters 380 (2003) 337–341

in-plane bending mode of bg symmetry strongly

involving the methyl groups of p-xylene cation.

The corresponding value of this vibration in the S1

state has been reported to be 369 cm�1. The other

two intense bands at 801 and 1247 cm�1 are as-

signed t6b and t7a. The t6b mode is a radial skeletal

vibration of bg symmetry and the t7a mode is es-

sentially a C–CH3 stretching mode of ag symmetryof p-xylene cation. The two weak bands at 471 and

1187 cm�1 are assigned to t6a and t9a. The t6amode is also a radial skeletal vibration of agsymmetry and t9a is a C–H in plane bending mode

of ag symmetry of the p-xylene cation. A double

peak band at 990 and 1003 cm�1 is assigned to t1,which is a radial skeletal or ring breathing mode of

ag symmetry of p-xylene. The splitting of 13 cm�1

is probably due to a Fermi resonance. A weak

band at 341 cm�1 above the 00 band and its pro-

gression can be observed in Fig. 3. There is no

corresponding mode observed in the S1 state of

p-xylene. The ab initio calculation shows that this

mode can be assigned to t3, which is a twisting

vibration between benzene ring and its substitu-

ents. All the peak positions and their assignmentsare listed Table 1.

From Table 1, seven vibrational frequencies ofthe p-xylene cation are obtained. The vibrational

frequencies are compared with the values obtained

from ab initio calculations in Table 2. These cal-

culated values are in good agreement with the

Table 2

Vibrational frequencies of p-xylene cation

Mode Frequency

(cm�1)

Ab initio

calculation (cm�1)aDeviation

(%)

3 341 366 7.4

9b 440 404 )8.26a 471 526 11.7

6b 801 782 )2.41 990 995 0.4

9a 1187 1130 )4.17a 1247 1192 )4.3aUsing GAUSSIANAUSSIAN 98 with the basis set 6-31G**.

B. Zhang et al. / Chemical Physics Letters 380 (2003) 337–341 341

experimental values within about 10%, which isadequate to tentatively assign the vibrational fea-

tures of the ion ground state spectrum.

4. Conclusion

We have presented new spectroscopic data on

p-xylene cation. Vibrational structure could be re-solved for p-xylene cation and vibrational frequen-

cies of the ionic ground state have been determined.

These optically active vibrations include in-plane

bending, radial skeletal, C–CH3 stretching, C–H in

plane bending, and ring breathing vibrations. The

results from this experimental and theoretical ap-

proach help to gainmore insights into the properties

of the p-xylene ion which are important for the un-derstanding of ion molecule reactions.

Acknowledgements

This work was supported by the Deutsche

Forschungsgemeinschaft SFB 377 and National

Nature Science Foundation of China 29973039 isgratefully acknowledged.

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