- Email: [email protected]
Ion trap method combined with two-color laser spectroscopy of supersonic molecular beams: Photodissociation of trapped C6H5Cl+
Volume 166, number 5,6
CHEMICAL PHYSICS LETTERS
9 March 1990
ION TRAP METHOD COMBINED WITH TWO-COLOR LASER SPECTROSCOPY OF SUPERSONIC MOLECULAR BEAMS: PHOTODISSOCL4TION OF TRAPPED CsH&l+ Naohiko
MIKAMI,
Yasushi
MIYATA,
Shin
SAT0 and Toshiki SASAKI
Departmentof Chemistry,Facultyof Science, TohokuUniversity,Sendai 980, Japan Received 24 December 1989
The cylindrical ion trap (UT) method has been applied to two-color laser spectroscopy in combination with supersonic moleo ular beams. Ions generated by resonant multiphoton ionization of chlorobenzene with a nanosecond-pulsed dye laser were stored in a CIT cell for more than 10 ms after the ionization. With this technique the photodissociation of the selectively trapped cblorobenzene cation was observed by using a second dye laser. The yield spectrum of the dissociation to phenyl radical cation was obtained under the ideal condition of no background fragments.
1. Introduction Ion trap methods have long been used in studies of chemical processes of positive and negative molecular ions in gas phases [ 1,2]. The ion cyclotron resonance (ICR) technique [ 31 is one of the methods which enable us to investigate processes occurring in molecular ions on a time scale of 10m3to 10’ s. Using the ICR method, Dunbar and Asamoto [ 46] have characterized the collisional and the collisionless thermalization of chlorobenzene ion and also observed the infrared radiative relaxation of iodobenzene ion. Recently they have developed time-resolved photodissociation of molecular cations in combination with TOF mass spectrometry [ 71. Brauman and Wetzel [8] have developed electron photodetachment threshold spectroscopy of molecular anions trapped with the ICR technique and investigated electronic excited states and autodetachment processes of anions. The cylindrical ion trap (CIT) technique [ 9 ] is known to be another efficient ion storage method. Time-resolved photoionization mass spectrometry combined with a CIT cell has been employed by Lifshitz and coworkers [ 10,111 in studies of dissociation of metastable polyatomic molecular cations. They have extensively investigated kinetic shifts of dissociation processes occurring in cations of benzene derivatives and of other compounds [ 12,131. 470
In this paper we described the ion trap method in conjunction with supersonic molecular beams and resonance multiphoton ionization (MPI). The CIT technique is employed because of its simplicity. Ions of chlorobenzene cation generated by resonant MPI with a tunable dye laser are stored in a CIT cell and are subjected to photodissociation with another tunable dye laser. The selective storage of particular ionic species is possible with this method so that photodissociation studies of the ions are carried out without being disturbed by fragment ions generated at the same time of the MPI process. With the application to spectroscopic and dynamical studies of molecular cluster ions in mind, we will discuss the characteristics of the CIT method combined with supersonic molecular beams.
2. Experimental Techniques of the CIT method and its general characteristics have been described in detail by authors of several groups [ 9,141. Here we describe our CIT cell briefly. The cell consists of three electrodes; a cylinder electrode and two end cap electrodes. The inner dimensions of the cell are 40 mm in diameter and 28 mm in height. The cylinder electrode was “potentialized” by an ac voltage whose frequency was in a range of 100-l 50 kHz and whose amplitude was
0009-2614/90/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland)
Volume 166, number 5,6
CHEMICAL PHYSICS LElTERS
less than 50 V. The end cap electrodes were kept grounded so that an oscillating electric field of approximately hyperbolic was generated inside the cell. On the wall of the cylinder electrode four holes were drilled for inlets and outlets of molecular beam and of laser beams. In ordinary CIT method a dc voltage may also be used as a bias onto the ac potential in order to improve mass separability for trapped ions. In this work, however, we did not use the dc bias because of a slightly asymmetric structure of the cell originating from the inequivalent end caps described below. The end cap on the top of the cell was made of stainless steel mesh so that after injecting a pulsed molecular beam the residual vapor in the cell was efficiently evacuated. Though this end cap was normally grounded, it was also used as a repeller electrode, if necessary, which pushed ions out from the cell by a pulsed repeller voltage ( + 10 V with a duration of 1ps ) . The other end cap electrode was made of stainless steel plate and was always grounded. At the center of this electrode a hole (10 mm in diameter) was opened for the ion exit. Ions ejected through the exit were collimated by an ion lens to a Q-pole mass filter and were detected by an electron multiplier. Fig. 1 shows a schematic of the setup. A pulsed supersonic free expansion of a gaseous mixture of C6H5Cl vapor at room temperature and
9 March 1990
He of about 2 atm was generated by a pulsed nozzle system [ 151 with an orifice of 0.8 mm in diameter. The free jet was skimmed by a skimmer of 1 mm diameter placed 10 mm downstream of the nozzle. The resulting pulsed molecular beam with a duration of about 400 ps was brought into the CIT cell through one of the inlet holes. The ultracold chlorobenzene molecules in the beam were photoionized with a dye laser at the center of the cell, where the laser beam and the molecular beam were crossed. Since the translational energy of the neutral molecules in the beam is of the order of several tens of millielectronvolt, the photoionized molecules in the cell are easily trapped by the applied electric field used. The UV light for the resonance MPI of C&Cl was generated by frequency doubling of an excimer laserpumped dye laser with Coumarin 540A dye in methanol. The UV output of about 100 pJ with 12 ns pulse width was focused by a lens onto the center of the CIT cell through the other inlet which is placed at the right angle from the inlet hole for the molecular beam. For the photodissociation of the trapped ion, frequency-doubled outputs of a Nd:YAG laserpumped dye laser were used. Without being focused the UV output of about 1 mJ pulse of 10 ns width was introduced into the cell from the opposite direction of the first UV light beam. Delayed pulse generators were used to provide precise timings among the pulsed molecular beam, the ionization laser, the photodissociation laser and the pulsed repeller.
3. Results and discussion
Fig. 1. Schematic of the apparatus. Npulsed nozzle, Sxkimmer, C:CIT cell. Lion lens, Q:quadrupole mass filter, Ezelectron multiplier. Laser beams come from the direction perpendicular to the plane. Three chambers are evacuated to: p,=5 x lo-‘, p,=2~10-~,p,=I~lO-~Torr.
Fig. 2A shows the mass spectrum obtained just after the two-photon ionization of chlorobenzene, where the laser wavelength was tuned at 269.9 nm for the excitation of the O-Oband of the S , c Sotransition [16,17].Themasspeaksat 112and 114amu are due to chlorobenzene cation, CsHsCl+, with isotopes of Cl atom of 35 and 37 amu, respectively. As usually observed in mass spectra, the peak at 76 + 1 amu is due to the phenyl radical ion [ 16 1, C,H: or to C6Hz ion and the peak at 50 &2 amu may be due to C4Hz ions, representing that, as soon as &H&I is ionized, the successive fragmentation occurs un471
CHEMICAL PHYSICS LETTERS
Volume 166, number 56
A) h,=269.9 nm
‘IT
: 2.0msec f;dOKHZ
\kF'35V
I 1, =28l.Onm Td = 2.0msec fg130KHz vg35v
Al-50
100 MASS NUMBER/w
150
Fig. 2. Mass spectra after tbe photoionization of CeH,CI. (A) Spectrum obtained with zero delay time after the two-photon ionization with 1., = 269.9 nm. (B) Spectrum obtained by applying the pulsed repeller with a delay time T,=2 ms from the ionization. The frequency& and the voltage VW of the ac-potential for the CIT cell are shown. (C) Spectrum obtained after the photolysis of the trapped CsHsCl+ by &=281.0 nm with T,=2 ms from the 1, pulse. The trapping conditions are the same as those in (B). The intense peak at 77 amu is due to the major daughter ion &Hz. The minor daughter ion appears at 50 + 2 amu because of the photolysis with high photon energy of L2.
472
9 March 1990
der an intense photon field of the laser for MPI. Since the apparent yield of the daughter ion C6H: depends upon the yield of the parent ion C6H5Clf, the yield spectrum of the former ion turns out to be approximately the same as the MPI spectrum of the latter. This well-known difficulty in observing the yield spectrum of the daughter ion has been removed with the help of the ion trap method. When the electric field of the CIT cell was properly adjusted so as to store the parent ion, the mass spectrum obtained after a storage time of 2 ms from the ionization is shown in Iig. 2B. The fragment ions produced at the moment of the ionization were clearly swept out under the unfavourable storage condition for them. The residence time or the storage time of the trapped ion is one of the important characteristics of the CIT cell [ 181 because its cylindrical electrode generates only an approximated hyperbolic potential for the ion trapping. It was measured by the pulsed repeller method, the relative ion current was monitored as a function of the delay time between pulses of the ionizing laser of the repeller. Although the definition of the residence time may not be unequivocal, it may be represented by the delay time giving a half of the intensity at the zero delay. The residence time of the parent ion was found to be longer than 10 ms which was the maximum of the delay time in our pulsed repeller unit. This means that the initially prepared ions are efficiently stored in the cell in spite of its quite simple shape of the electrodes. This also suggests that the pseudo-hyperbolic potential is effective to trap ions created near the center of the cell by means of the molecular beam and the MPI methods. The spilling of ions from the cell, however, was seen when the photoionization created too many ions within a small space. Space charge effect of the high density ion cloud seems to be due to the spilling. In the present work, the photon density of the ionizing laser was kept low enough so as to avoid the ion cloud explosion. The photodissociation of the trapped ion was readily observed by introducing the second laser passing through the center of the CIT cell. Fig. 2C represents the mass spectrum showing the daughter ion generated by the photolysis of the parent ion. The laser pulse was led into the cell with a delay time of 2 ms after the photoionization by the first laser. As shown in fig. 2B, the parent ion is the only resident
Volume 166, number $6
CHEMICAL PHYSICS LETTERS
species in the cell after the ionization. Therefore, the mass spectrum shown in fig. 2C clearly represents the following dissociation process: CgHsC1++CgH;’ +Cl’ without being disturbed by the by-products upon the ionization. An interesting characteristic of the CIT cell is the following: when the daughter ion was generated by the laser photolysis it was automatically ejected out from the cell without using any extraction technique as far as the trapping condition is critically tuned for the parent. The reason is that the daughter ions were unstable at the saddle point of the oscillating electric field where the parent is stable and that the cylindrically symmetric potential eliminates the unstable ions along the center line of the cell where the ion exit is opened. This characteristic is quite useful for the spectroscopic studies of molecular ions or of cluster ions with monitoring photofragments, that is, the fragment monitor spectroscopy of ions. Fig. 3 shows the yield spectrum of C,H: from the trapped C6H&l+ as a function of wavelength of the photodissociation laser. The gradual increase starting from 330 nm and the steep rise from 3 17 nm are found, suggesting that the dissociation threshold of PHO;tllENERGY/eV
3.70
390
t
a d
a
I 041 335
. *-4 T
=
q330. *
4
I
.
I
320 325 LASER WAVELENGTH/nm
I
315
Fig. 3. Yield spectrum of C,H: from the trapped C&Cl+. The yield is normalized with respect to the power spectrum of the dye laser used. The arrow indicates the reported value of the dissociation threshold [ 19 1.
9 March 1990
the process should be in a range between 3.757 and 3.910 eV. The dissociation threshold has recently been reported to be at 3.81 eV [ 191 lying above the ionization potential (IP) of C6HSCl found at 9.066 eV [ 201. A fairly good agreement of the result with the reported threshold is seen, although the onset of the yield in the present work starts from about 60 meV below the reported value. The gradual onset of the yield spectrum is understood by considering MPI process of CsHSC1.Since the two-photon energy of the ionizing light was 9.187 eV, the difference between this and IP of C6H&1 prepares the ion and electron with an excess energy of about 0.12 1 eV. Some of the excess energy is taken away from the system by a kinetic energy of the ejected electron and the rest is stored by the hot ion which is vibrationally excited. Although the hot ion can be cooled down by the radiative relaxation of infrared radiation during such a long residence time in the cell, the relaxation rate may not be so fast as to complete the cooling [ 51. Thus we conclude thatthe vague onset observed in the dissociation yield spectrum is due to vibrationally hot ions generated by the one-color, twophoton ionization.
4. Concluding remarks The present work demonstrates that the CIT method is quite effective in storing ions generated by MPI of supersonic molecular beams. Several extensions of this method emerge from this success; twocolor MPI is available to pump jet-cooled molecules to their adiabatic IP, so that ultracold ions can be trapped with the CIT cell. Then, spectroscopy and the photodynamics of the ultracold molecular ions are possible. Secondly, since fragmentation occurs usually due to predissociation from discrete states (vibronic or rovibronic) of the parent ions, the yield spectrum of a daughter ion represents the spectrum of the trapped parent ion, that is, thefragment-monitor spectrum is available for the spectroscopy of molecular ions. This spectroscopy is also useful to investigate excited states of molecular cluster ions. An abundance of molecular clusters in supersonic molecular beams can be ionized and among others a cluster ion with a particular size can be selectively trapped. Photoabsorption of the trapped cluster ion 473
Volume 166, number 5,6
CHEMICAL PHYSICS LETTERS
leads to fragmentation to smaller size cluster ion or monomer ion so that the fragment ion monitor spectroscopy is now applicable with the CIT method. Studies of dimers of benzene, of fluorobenzene and of other aromatic compounds are in progress in our laboratory.
Acknowledgement
The authors thank Professor Mitsuo Ito for stimulating discussions and for continuous encouragement. This work was supported by Grant-in-Aids from the Ministry of Education Japan (No. 63540363 and No. 01470014).
References [ 1] J.L. Beauchamp, L.R. Anders and I.D. Baldeschwieler, J. Am. Chem. Sot. 89 (1967) 4569. [2] J.I. Brauman and L.K. Blair, J. Am. Chem. Sot. 92 (1970) 5986. [ 31 J.R. Beauchamp, Ann. Rev. Phys. Chem. 22 (197 1) 527. [4] R.C. Dunbar, Chem. Phys. Letters 125 (1986) 543.
474
9 March 1990
[5] R.C. Dunbar, J. Phys. Chem. 91 (1987) 2801. [6] B. Asamoto and R.C. Dunbar, J. Phys. Chem. 91 (1987) 2804. [7] R.C. Dunbar and G.H. Weddle, J. Phys. Chem. 92 (1988) 5706. [ 8 ] D.M. Wetzeland J.I. Brauman, Chem. Rev. 87 ( 1987) 607. [9] R.E. Mather, R.M. Waldren, J.F.J. Todd and R.E. March, Intern. J. Mass Spectrom. Ion Phys. 33 ( 1980) 201. [lo] Y. Malinovich, R. Arakawa, G. Haase and C. Lifshitz, .I. Phys. Chem. 89 (1985) 2253. [ 1 1 ] Y. Malinovich and C. Lifshitz, J. Phys. Chem. 90 ( 1986) 2200. [ 121 C. Lifshitz, J. Phys. Chem. 86 ( 1982) 606. [ 131 C. Lifshitz, S. Gefen and R. Arakawa, J. Phys. Chem. 88 (1984) 4242. [ 141 M-N. Benilan and C. Audoin, Intern. J. Mass Spectrom. Ion Phys. I1 (1973) 421. [ 151 N. Mikami, I. Suzuki and A. Okabe, J. Phys. Chem. 91 (1987) 5242. [ 161J.L. Durant, D.M. Rider, S.L. Anderson, ED. Proch and R.N.Zare, J.Chem.Phys.80 (1984) 1817. [ 171 T. Maeyama and N. Mikami, J. Am. Chem. Sot. 110 ( 1988) 7238. [ 18) J. Yoda and K. Sugiyama, Japan. J. Appl. Phys. 26 (1987) 1780. [ 19] X. Ripoche, I. Dimicoli, J. LeCalve, F. Piuzzi and R. Batter, Chem. Phys. 124 (1988) 305. [ 20 ] H.M. Rosenstock, K. Drazl, B.W. Steiner and J.T. Herron, J. Phys. Chem. Ref. Data 6 Suppl. 1 (1977).
CHEMICAL PHYSICS LETTERS
9 March 1990
ION TRAP METHOD COMBINED WITH TWO-COLOR LASER SPECTROSCOPY OF SUPERSONIC MOLECULAR BEAMS: PHOTODISSOCL4TION OF TRAPPED CsH&l+ Naohiko
MIKAMI,
Yasushi
MIYATA,
Shin
SAT0 and Toshiki SASAKI
Departmentof Chemistry,Facultyof Science, TohokuUniversity,Sendai 980, Japan Received 24 December 1989
The cylindrical ion trap (UT) method has been applied to two-color laser spectroscopy in combination with supersonic moleo ular beams. Ions generated by resonant multiphoton ionization of chlorobenzene with a nanosecond-pulsed dye laser were stored in a CIT cell for more than 10 ms after the ionization. With this technique the photodissociation of the selectively trapped cblorobenzene cation was observed by using a second dye laser. The yield spectrum of the dissociation to phenyl radical cation was obtained under the ideal condition of no background fragments.
1. Introduction Ion trap methods have long been used in studies of chemical processes of positive and negative molecular ions in gas phases [ 1,2]. The ion cyclotron resonance (ICR) technique [ 31 is one of the methods which enable us to investigate processes occurring in molecular ions on a time scale of 10m3to 10’ s. Using the ICR method, Dunbar and Asamoto [ 46] have characterized the collisional and the collisionless thermalization of chlorobenzene ion and also observed the infrared radiative relaxation of iodobenzene ion. Recently they have developed time-resolved photodissociation of molecular cations in combination with TOF mass spectrometry [ 71. Brauman and Wetzel [8] have developed electron photodetachment threshold spectroscopy of molecular anions trapped with the ICR technique and investigated electronic excited states and autodetachment processes of anions. The cylindrical ion trap (CIT) technique [ 9 ] is known to be another efficient ion storage method. Time-resolved photoionization mass spectrometry combined with a CIT cell has been employed by Lifshitz and coworkers [ 10,111 in studies of dissociation of metastable polyatomic molecular cations. They have extensively investigated kinetic shifts of dissociation processes occurring in cations of benzene derivatives and of other compounds [ 12,131. 470
In this paper we described the ion trap method in conjunction with supersonic molecular beams and resonance multiphoton ionization (MPI). The CIT technique is employed because of its simplicity. Ions of chlorobenzene cation generated by resonant MPI with a tunable dye laser are stored in a CIT cell and are subjected to photodissociation with another tunable dye laser. The selective storage of particular ionic species is possible with this method so that photodissociation studies of the ions are carried out without being disturbed by fragment ions generated at the same time of the MPI process. With the application to spectroscopic and dynamical studies of molecular cluster ions in mind, we will discuss the characteristics of the CIT method combined with supersonic molecular beams.
2. Experimental Techniques of the CIT method and its general characteristics have been described in detail by authors of several groups [ 9,141. Here we describe our CIT cell briefly. The cell consists of three electrodes; a cylinder electrode and two end cap electrodes. The inner dimensions of the cell are 40 mm in diameter and 28 mm in height. The cylinder electrode was “potentialized” by an ac voltage whose frequency was in a range of 100-l 50 kHz and whose amplitude was
0009-2614/90/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland)
Volume 166, number 5,6
CHEMICAL PHYSICS LElTERS
less than 50 V. The end cap electrodes were kept grounded so that an oscillating electric field of approximately hyperbolic was generated inside the cell. On the wall of the cylinder electrode four holes were drilled for inlets and outlets of molecular beam and of laser beams. In ordinary CIT method a dc voltage may also be used as a bias onto the ac potential in order to improve mass separability for trapped ions. In this work, however, we did not use the dc bias because of a slightly asymmetric structure of the cell originating from the inequivalent end caps described below. The end cap on the top of the cell was made of stainless steel mesh so that after injecting a pulsed molecular beam the residual vapor in the cell was efficiently evacuated. Though this end cap was normally grounded, it was also used as a repeller electrode, if necessary, which pushed ions out from the cell by a pulsed repeller voltage ( + 10 V with a duration of 1ps ) . The other end cap electrode was made of stainless steel plate and was always grounded. At the center of this electrode a hole (10 mm in diameter) was opened for the ion exit. Ions ejected through the exit were collimated by an ion lens to a Q-pole mass filter and were detected by an electron multiplier. Fig. 1 shows a schematic of the setup. A pulsed supersonic free expansion of a gaseous mixture of C6H5Cl vapor at room temperature and
9 March 1990
He of about 2 atm was generated by a pulsed nozzle system [ 151 with an orifice of 0.8 mm in diameter. The free jet was skimmed by a skimmer of 1 mm diameter placed 10 mm downstream of the nozzle. The resulting pulsed molecular beam with a duration of about 400 ps was brought into the CIT cell through one of the inlet holes. The ultracold chlorobenzene molecules in the beam were photoionized with a dye laser at the center of the cell, where the laser beam and the molecular beam were crossed. Since the translational energy of the neutral molecules in the beam is of the order of several tens of millielectronvolt, the photoionized molecules in the cell are easily trapped by the applied electric field used. The UV light for the resonance MPI of C&Cl was generated by frequency doubling of an excimer laserpumped dye laser with Coumarin 540A dye in methanol. The UV output of about 100 pJ with 12 ns pulse width was focused by a lens onto the center of the CIT cell through the other inlet which is placed at the right angle from the inlet hole for the molecular beam. For the photodissociation of the trapped ion, frequency-doubled outputs of a Nd:YAG laserpumped dye laser were used. Without being focused the UV output of about 1 mJ pulse of 10 ns width was introduced into the cell from the opposite direction of the first UV light beam. Delayed pulse generators were used to provide precise timings among the pulsed molecular beam, the ionization laser, the photodissociation laser and the pulsed repeller.
3. Results and discussion
Fig. 1. Schematic of the apparatus. Npulsed nozzle, Sxkimmer, C:CIT cell. Lion lens, Q:quadrupole mass filter, Ezelectron multiplier. Laser beams come from the direction perpendicular to the plane. Three chambers are evacuated to: p,=5 x lo-‘, p,=2~10-~,p,=I~lO-~Torr.
Fig. 2A shows the mass spectrum obtained just after the two-photon ionization of chlorobenzene, where the laser wavelength was tuned at 269.9 nm for the excitation of the O-Oband of the S , c Sotransition [16,17].Themasspeaksat 112and 114amu are due to chlorobenzene cation, CsHsCl+, with isotopes of Cl atom of 35 and 37 amu, respectively. As usually observed in mass spectra, the peak at 76 + 1 amu is due to the phenyl radical ion [ 16 1, C,H: or to C6Hz ion and the peak at 50 &2 amu may be due to C4Hz ions, representing that, as soon as &H&I is ionized, the successive fragmentation occurs un471
CHEMICAL PHYSICS LETTERS
Volume 166, number 56
A) h,=269.9 nm
‘IT
: 2.0msec f;dOKHZ
\kF'35V
I 1, =28l.Onm Td = 2.0msec fg130KHz vg35v
Al-50
100 MASS NUMBER/w
150
Fig. 2. Mass spectra after tbe photoionization of CeH,CI. (A) Spectrum obtained with zero delay time after the two-photon ionization with 1., = 269.9 nm. (B) Spectrum obtained by applying the pulsed repeller with a delay time T,=2 ms from the ionization. The frequency& and the voltage VW of the ac-potential for the CIT cell are shown. (C) Spectrum obtained after the photolysis of the trapped CsHsCl+ by &=281.0 nm with T,=2 ms from the 1, pulse. The trapping conditions are the same as those in (B). The intense peak at 77 amu is due to the major daughter ion &Hz. The minor daughter ion appears at 50 + 2 amu because of the photolysis with high photon energy of L2.
472
9 March 1990
der an intense photon field of the laser for MPI. Since the apparent yield of the daughter ion C6H: depends upon the yield of the parent ion C6H5Clf, the yield spectrum of the former ion turns out to be approximately the same as the MPI spectrum of the latter. This well-known difficulty in observing the yield spectrum of the daughter ion has been removed with the help of the ion trap method. When the electric field of the CIT cell was properly adjusted so as to store the parent ion, the mass spectrum obtained after a storage time of 2 ms from the ionization is shown in Iig. 2B. The fragment ions produced at the moment of the ionization were clearly swept out under the unfavourable storage condition for them. The residence time or the storage time of the trapped ion is one of the important characteristics of the CIT cell [ 181 because its cylindrical electrode generates only an approximated hyperbolic potential for the ion trapping. It was measured by the pulsed repeller method, the relative ion current was monitored as a function of the delay time between pulses of the ionizing laser of the repeller. Although the definition of the residence time may not be unequivocal, it may be represented by the delay time giving a half of the intensity at the zero delay. The residence time of the parent ion was found to be longer than 10 ms which was the maximum of the delay time in our pulsed repeller unit. This means that the initially prepared ions are efficiently stored in the cell in spite of its quite simple shape of the electrodes. This also suggests that the pseudo-hyperbolic potential is effective to trap ions created near the center of the cell by means of the molecular beam and the MPI methods. The spilling of ions from the cell, however, was seen when the photoionization created too many ions within a small space. Space charge effect of the high density ion cloud seems to be due to the spilling. In the present work, the photon density of the ionizing laser was kept low enough so as to avoid the ion cloud explosion. The photodissociation of the trapped ion was readily observed by introducing the second laser passing through the center of the CIT cell. Fig. 2C represents the mass spectrum showing the daughter ion generated by the photolysis of the parent ion. The laser pulse was led into the cell with a delay time of 2 ms after the photoionization by the first laser. As shown in fig. 2B, the parent ion is the only resident
Volume 166, number $6
CHEMICAL PHYSICS LETTERS
species in the cell after the ionization. Therefore, the mass spectrum shown in fig. 2C clearly represents the following dissociation process: CgHsC1++CgH;’ +Cl’ without being disturbed by the by-products upon the ionization. An interesting characteristic of the CIT cell is the following: when the daughter ion was generated by the laser photolysis it was automatically ejected out from the cell without using any extraction technique as far as the trapping condition is critically tuned for the parent. The reason is that the daughter ions were unstable at the saddle point of the oscillating electric field where the parent is stable and that the cylindrically symmetric potential eliminates the unstable ions along the center line of the cell where the ion exit is opened. This characteristic is quite useful for the spectroscopic studies of molecular ions or of cluster ions with monitoring photofragments, that is, the fragment monitor spectroscopy of ions. Fig. 3 shows the yield spectrum of C,H: from the trapped C6H&l+ as a function of wavelength of the photodissociation laser. The gradual increase starting from 330 nm and the steep rise from 3 17 nm are found, suggesting that the dissociation threshold of PHO;tllENERGY/eV
3.70
390
t
a d
a
I 041 335
. *-4 T
=
q330. *
4
I
.
I
320 325 LASER WAVELENGTH/nm
I
315
Fig. 3. Yield spectrum of C,H: from the trapped C&Cl+. The yield is normalized with respect to the power spectrum of the dye laser used. The arrow indicates the reported value of the dissociation threshold [ 19 1.
9 March 1990
the process should be in a range between 3.757 and 3.910 eV. The dissociation threshold has recently been reported to be at 3.81 eV [ 191 lying above the ionization potential (IP) of C6HSCl found at 9.066 eV [ 201. A fairly good agreement of the result with the reported threshold is seen, although the onset of the yield in the present work starts from about 60 meV below the reported value. The gradual onset of the yield spectrum is understood by considering MPI process of CsHSC1.Since the two-photon energy of the ionizing light was 9.187 eV, the difference between this and IP of C6H&1 prepares the ion and electron with an excess energy of about 0.12 1 eV. Some of the excess energy is taken away from the system by a kinetic energy of the ejected electron and the rest is stored by the hot ion which is vibrationally excited. Although the hot ion can be cooled down by the radiative relaxation of infrared radiation during such a long residence time in the cell, the relaxation rate may not be so fast as to complete the cooling [ 51. Thus we conclude thatthe vague onset observed in the dissociation yield spectrum is due to vibrationally hot ions generated by the one-color, twophoton ionization.
4. Concluding remarks The present work demonstrates that the CIT method is quite effective in storing ions generated by MPI of supersonic molecular beams. Several extensions of this method emerge from this success; twocolor MPI is available to pump jet-cooled molecules to their adiabatic IP, so that ultracold ions can be trapped with the CIT cell. Then, spectroscopy and the photodynamics of the ultracold molecular ions are possible. Secondly, since fragmentation occurs usually due to predissociation from discrete states (vibronic or rovibronic) of the parent ions, the yield spectrum of a daughter ion represents the spectrum of the trapped parent ion, that is, thefragment-monitor spectrum is available for the spectroscopy of molecular ions. This spectroscopy is also useful to investigate excited states of molecular cluster ions. An abundance of molecular clusters in supersonic molecular beams can be ionized and among others a cluster ion with a particular size can be selectively trapped. Photoabsorption of the trapped cluster ion 473
Volume 166, number 5,6
CHEMICAL PHYSICS LETTERS
leads to fragmentation to smaller size cluster ion or monomer ion so that the fragment ion monitor spectroscopy is now applicable with the CIT method. Studies of dimers of benzene, of fluorobenzene and of other aromatic compounds are in progress in our laboratory.
Acknowledgement
The authors thank Professor Mitsuo Ito for stimulating discussions and for continuous encouragement. This work was supported by Grant-in-Aids from the Ministry of Education Japan (No. 63540363 and No. 01470014).
References [ 1] J.L. Beauchamp, L.R. Anders and I.D. Baldeschwieler, J. Am. Chem. Sot. 89 (1967) 4569. [2] J.I. Brauman and L.K. Blair, J. Am. Chem. Sot. 92 (1970) 5986. [ 31 J.R. Beauchamp, Ann. Rev. Phys. Chem. 22 (197 1) 527. [4] R.C. Dunbar, Chem. Phys. Letters 125 (1986) 543.
474
9 March 1990
[5] R.C. Dunbar, J. Phys. Chem. 91 (1987) 2801. [6] B. Asamoto and R.C. Dunbar, J. Phys. Chem. 91 (1987) 2804. [7] R.C. Dunbar and G.H. Weddle, J. Phys. Chem. 92 (1988) 5706. [ 8 ] D.M. Wetzeland J.I. Brauman, Chem. Rev. 87 ( 1987) 607. [9] R.E. Mather, R.M. Waldren, J.F.J. Todd and R.E. March, Intern. J. Mass Spectrom. Ion Phys. 33 ( 1980) 201. [lo] Y. Malinovich, R. Arakawa, G. Haase and C. Lifshitz, .I. Phys. Chem. 89 (1985) 2253. [ 1 1 ] Y. Malinovich and C. Lifshitz, J. Phys. Chem. 90 ( 1986) 2200. [ 121 C. Lifshitz, J. Phys. Chem. 86 ( 1982) 606. [ 131 C. Lifshitz, S. Gefen and R. Arakawa, J. Phys. Chem. 88 (1984) 4242. [ 141 M-N. Benilan and C. Audoin, Intern. J. Mass Spectrom. Ion Phys. I1 (1973) 421. [ 151 N. Mikami, I. Suzuki and A. Okabe, J. Phys. Chem. 91 (1987) 5242. [ 161J.L. Durant, D.M. Rider, S.L. Anderson, ED. Proch and R.N.Zare, J.Chem.Phys.80 (1984) 1817. [ 171 T. Maeyama and N. Mikami, J. Am. Chem. Sot. 110 ( 1988) 7238. [ 18) J. Yoda and K. Sugiyama, Japan. J. Appl. Phys. 26 (1987) 1780. [ 19] X. Ripoche, I. Dimicoli, J. LeCalve, F. Piuzzi and R. Batter, Chem. Phys. 124 (1988) 305. [ 20 ] H.M. Rosenstock, K. Drazl, B.W. Steiner and J.T. Herron, J. Phys. Chem. Ref. Data 6 Suppl. 1 (1977).