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Two-dimensional photoelectron imaging of state-selected iodine atom by (2+1) resonance-enhanced multiphoton ionization
<'i .4/c
29 September1995
' 77
CHEMICAL PHYSICS LETTERS
ELSEVIER
Chemical PhysicsLetters 244 (1995) 183-187
Two-dimensional photoelectron imaging of state-selected iodine atom by (2 + 1) resonance-enhanced multiphoton ionization Wee Kyung Kang, Yong Shin Kim, Kyung-HoonJung
*
Center for Molecular Science and Department of Chemistry, Korea Advanced Institute of Science and Technology, Taeduck Science Town, Taejon 305-701, South Korea
Received 29 June 1995
Abstract
The state-selective photoelectron imaging technique is applied to investigate the angular distribution of ejected electrons and the electronic state of the atomic ion produced by (2 + 1) resonance-enhanced multiphoton ionization of iodine atom in the photolysis of CH3I. Strong anisotropic distribution behaviors have been depicted on two electron images observed, one • o . , • 4 o . . . . . via the two-photon resonance 2 D5/z state from ground state ~odme and, another via D~/z from spin-orbit excited ~odlne. . . . . . . . 2 o . . . . . . The configuration of core electrons ~s preserved m the ~omzat~on from the Ds/z mtermedmte state, whale the ~omzatlon from the D~/2 state shows two distinct ionization paths, a preserved and a non-preserved path of core electrons.
1. Introduction
Resonance-enhanced multiphoton ionization coupled with photoelectron spectroscopy (REMPI-PES) is a powerful technique to study photoionization dynamics of atomic fragments produced from the photodissociation of molecules [1-8]. Physical quantities concerned in the PES are the kinetic energy and angular distribution o f electrons ejected from neutral states of atomic fragments. The kinetic energy of electron provides the information of internal state of product ion; e.g. the electronically excited state of atomic ion and vibrational state of molecular
* Corresponding author.
ion. The angular distribution of photoelectrons allows the characterization of aligned intermediate states by resonantly absorbed multiphoton and the correlation between the neutral excited state and the ionic state [9-13]. The reactivity of ionic states and the preparation of state-selected ions have received much attention for ion-molecule investigation and its application to plasma and semiconductor process. The conventional PES is based on the kinetic energy measurement of an ejected photoelectron at a fixed polarization angle with respect to the propagation direction of electron. Charged particle imaging techniques coupled with REMPI [14] have the advantage of simultaneous observation and characterization of the angular and kinetic energy distribution of ions and electrons. In this Letter, we present a technique in which the
0009-2614/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0009-2614(95)00880-2
184
W.K. Kang et aL / Chemical Physics Letters 244 (1995) 183-187
three-dimensional spatial distribution of photoelectrons, ejected from iodine atom produced via the photodissociation of the CH3I molecule, has been projected onto two-dimensional surfaces. Atomic iodine, an open-shell atom, has been chosen as a prototype candidate for the production of stateselected ions because of its energy level structure [15]. The ionic state selectivity of iodine atom has been determined through ionization pathways of the intermediate state from the spatial distribution of photoelectrons.
2. Experimental The details of the imaging spectrometer have been described elsewhere for determining the spatial distribution of the iodine atom photofragmented from the CF3I molecule [16]. Photoelectron imaging is essentially the same as that of photoion except that a negative high voltage is applied to the repelling plate. To reduce the effect of ambient magnetic field
on photoelectron trajectories, a ix-metal shield is mounted inside of the time-of-flight tube and the ionization region. A pulsed molecular beam of 3% CH3I seeded in He is injected into the ionization region through a skimmer. Iodine atoms are produced by UV photolysis of CH3I. CH3I dissociates into two channels leading to either a methyl radical and ground state iodine (2P~/z), or a methyl radical and a spin-orbit excited iodine atom (2p?/2). The same laser pulse ionizes the iodine atom via twophoton resonant intermediate state. The UV laser light is produced by doubling the output from a nanosecond Nd:YAG pumped dye laser and is linearly polarized by a half-wave retardation plate. The REMPI spectrum is obtained by monitoring the iodine ion signal and scanning the laser wavelength. The photoelectron cloud expanded from the ionization point is projected onto a microchannel plate/phosphor screen detector at the resonant wavelength of iodine. The images are recorded with an electronic CCD camera (512 × 512 pixels). The CCD camera integrates the images from about 3000 laser
(a)
(b)
0,0 0,2
0,4
0,6
0,8
1.0
Fig. 1. Photoelectron images by (2 + 1) REMPI of: (a) ground state iodine at 304.67 nm and; (b) spin-orbit excited state iodine at 304.02 nm.
W.K. Kang et al. / Chemical Physics Letters 244 (1995) 183-187
shots and the blank image, obtained at non-resonant laser wavelength of iodine atom, is then subtracted.
185
(a) 304.67 nm 1.75 eV
3. Results and discussion
Fig. 1 shows the images of photoelectron ejected from ground state (a) and spin-orbit excited state (b) iodine atoms for alignment of the polarization of laser parallel to detector plate. The ground state iodine atoms are selectively ionized via two-photon o resonance to the (3p) 6p 2 Ds/2 state using 304.67 nm and the spin-orbit excited state iodine via the (3p) 6paD~/2 state using 304.02 nm [15]. Both electron images are anisotropic and depict typical polar-cap appearances. In addition the image from REMPI of excited state iodine atom exhibits an additional inner shell suggesting two ionization paths, i.e. the internal states of iodine ions. The correlation of atomic states in two-photon absorption with linearly polarized light is followed by the symmetry selection rules: AS = 0; A J = 0 , +1, +_2with J=O~--/---~J=l; A L = 0 , _+1, + 2 w i t h L = O ~ / ~ L = l ; a n d A I = 0 , _+2 [7]. While the two-photon transition, (3p) 6p 2D5/2o .__ 2 P~/2 is an allowed transition, the forbidden transition, (3p) 6p4D~/2 ~ ~.plo/e of spin-orbit excited iodine, violating the conservation rule of electron spin, can be described by configuration interaction in the iodine atom. The angular distribution of photoelectrons, obtained by multiphoton ionization utilizing a single linearly polarized laser, is expressed by [9-13] We(0) = Ea2kP2k(cos 0),
(1)
where Pzk(COS 0) is the Legendre polynomial of
order 2k, 0 the angle between light polarization and the photoelectron propagation vector, and azk, the coefficient, contains information on the partial wave character of the outgoing photoelectron in addition to alignment of the resonant intermediate state [12]. For (n + 1) REMPI via the spherically symmetric intermediate state, the expression is reduced to a 0 + a2Pz(cOs 0). The ratio A 2 = ao/a 2 represents the familiar asymmetry parameter /3 used in conventional description of single photon ionization [17]. The asymmetry parameter /3 from inverse Abel transformed image indicates a nearly parallel transi-
(b) 304.02 nm 1.88 eV
~
.
-200
-100
0
100
2;0
Pixel No. Fig. 2. Intensity cross sections through the center of the images: (a) ground state iodine at 304.67 nm and (b) excited state iodine at 304.02 nm.
tion and the intensity pattern with angle 0 shows the polar cap distribution [14]. These are contrasts to previous results that show A 2 coefficients of 1.36 for ionization of the S 4p3p0 state [5] and - 0 . 2 for N o 3p 2 $1/z states [2], respectively. Although no theoretical study has been reported on high-lying state ionization of iodine, the high anisotropy parameter of an electron ejected from the 6p valence shell of iodine is consistent with a maximum value for the electron energy lower than the Cooper minimum as described in the study of /3 for ionization from p subshells of ground-state atoms by Manson [18]. Fig. 2 gives intensity cross sections through the center of images taken at 304.67 nm (a) and 304.02 nm (b) along the direction of laser polarization. The separation of two peaks indicates velocity differences of outgoing electrons from two-photon resonant intermediate states. The velocity of the electron is obtained from v = d/2t where d is the diameter of the electron image and t the time-of-flight of the electron. Since the measurement of t involves large uncertainty, we determine the final state of the product iodine ion by comparing the separation of two electron peaks with the electron energy obtained from energy levels of iodine [15]. The ion states are assigned to the 3P2 state at 304.67 nm, and 3pj
186
W.K. Kang et al. / Chemical Physics Letters 244 (1995) 183-187
120000
1S 0
100000
~Dz
I(11) '7,
IP
_
80000
3p2 4D1/2 (3p)
2D;/z
6O000 304.67 nm
I(I y
~ 304.02 nm
400OO 10000"
2p30/2 (3p)
I(I )
Fig. 3. Schematic diagram depicting relevant energy levels of I(I) and I(II) with (2 + 1) REMPI of iodine atom at 304 nm.
( J = 0 or 1) for the outer-ring and 1D 2 states for the inner-ring at 304.02 nm, respectively. Their separations, 1 : 1.037 : 0.766, agree well with 1 : 1.025 (3P 0) or 1.047 (3P1):0.763 of electron velocity in the literature. The J splitting in the 3Po and 3P1 ionic states is unable to resolve the electron energy gap, 639 cm -1. An energy level diagram of ground and excited state iodines with (2 + 1) REMPI transition is illustrated in Fig. 3. Single-photon ionization involves the removal of a single electron from neutral species without changing the quantum numbers of any other electrons. For an ejected electron with l angular momentum, the change of L between neutral species and the ion is given by AL = 0, + 1. . . . _ I and the change of spin multiplicity becomes AS = _ 1. Since the ionization is achieved by absorbing an additional photon by an intermediate state, we treat these processes as single-photon ionization process thereon. Both ionizations to the 3P2 state from (3p) o state and to 3p0.1 from the (3p) 6p4D~/2 6p 2D5/2 state obey these selection rules and core electrons are preserved. However, the ionization to the ID 2 state o from (3p) 6p 4 D1/2 is a forbidden transition by the above selection rules and core electrons are not preserved. This is in contrast to the ionization of the valence shell np electron (n = 4-6) in excited S and P atoms [5,8]. Their ionizations are driven at ease by preservation of core electron configuration. The 1D 2 ion state selection of iodine may then be followed by the coupling of Rydberg electron and the ion core.
This effect has been found in transition metal atoms [19]. If (3p) 6p4D~/2 state iodine is excited to high-lying Rydberg state by a further photon absorption, 1D 2 state ion can be produced from Rydberg state by rapid radiationless relaxation, autoionization. In summary, photoelectron images of atomic iodine originating from the ground and spin-orbit excited state are first obtained via (2 + 1) REMPI to investigate the angular distribution of the ejected electron and the internal state of product ion in the dissociation of CH3I in the 304 nm region. Both electron images observed from ground and spin-orbit excited iodine atoms have shown cos20 dependence, which is characterized by one-photon ionization from the intermediate state, aligned spherically. The ionization of the (3p) 6p 4 D I / 2 intermediate state by the spin intercombination transition of the 2P~/2 state has the ion-core non preserving ionization path to 1D 2 iodine ion state. This is non-observed transition in p-electron ionization of N, S and P atoms via (2 + 1) REMPI driven by core-electron conservation.
Acknowledgement This work is supported by the Korea Science and Engineering Foundation, which is gratefully acknowledged.
References [1] S.T. Pratt, J.L. Dehmer and P.M. Dehmer, J. Chem. Phys. 67 (1985) 676. [2] S.T. Pratt, J.L. Dehmer and P.M. Dehmer, Phys. Rev. A 36 (1987) 1702. [3] S. Kroll, H. Lundberg, A. Persson and S. Svanberg, Phys. Rev. Letters 55 (1985) 284. [4] W.K. Bischel and L.E, Jusinski, Chem. Phys. Letters 120 (1985) 337. [5] J.R. Appling, M.R. Harbol, R.A. Edington and A.C. Goren, J. Chem. Phys. 97 (1992) 4041. [6] T.V. Venkitachalan and A.S. Rao, Appl. Phys. B 52 (1991) 102. [7] J. Steadman and T. Baer, J. Chem. Phys. 89 (1988) 5507. [8] M.R. Harbol, J.R. Appling and A.C. Goren, J. Chem. Phys. 101 (1994) 2659. [9] P. Lambropoulos, Advan. At. Mol. Phys. 12 (1976) 87. [10] M.P. Strand, J. Hansen, R.-L. Chien and R.S. Berry, Chem. Phys, Letters 59 (1978) 205.
W.K. Kang et al. / Chemical Physics Letters 244 (1995) 183-187
[11] S.N. Dixit and P.L. Lambropoulos, Phys. Rev. A 27 (1983) 168. [12] S.N. Dixit and V. Mckoy, J. Chem. Phys. 82 (1985) 3456. [13] J.R. Appling, M.G. White, W.J. Kessler, R. Fernandez and E.D. Polaikoff, J. Chem. Phys. 88 (1988) 2300. [14] D.W. Chandler and P.L. Houston, J. Chem. Phys. 87 (1987) 1445.
187
[15] C.E. Moore, Atomic energy levels, NSRDS-NBS, Vol. III, Circular No. 35 (US GPO, Washington, 1971) p. 170. [16] Y.S. Kim, W.K. Kang and K.-H. Jung, to be published. [17] J. Cooper and R.N. Zare, J. Chem. Phys. 48 (1968) 942. [18] S.T. Manson, J. Electron Spectry. 1 (1972) 413. [19] L. Sanders, A.D. Sappey and J.C. Weisshaar, J. Chem. Phys. 85 (1986) 6952.
29 September1995
' 77
CHEMICAL PHYSICS LETTERS
ELSEVIER
Chemical PhysicsLetters 244 (1995) 183-187
Two-dimensional photoelectron imaging of state-selected iodine atom by (2 + 1) resonance-enhanced multiphoton ionization Wee Kyung Kang, Yong Shin Kim, Kyung-HoonJung
*
Center for Molecular Science and Department of Chemistry, Korea Advanced Institute of Science and Technology, Taeduck Science Town, Taejon 305-701, South Korea
Received 29 June 1995
Abstract
The state-selective photoelectron imaging technique is applied to investigate the angular distribution of ejected electrons and the electronic state of the atomic ion produced by (2 + 1) resonance-enhanced multiphoton ionization of iodine atom in the photolysis of CH3I. Strong anisotropic distribution behaviors have been depicted on two electron images observed, one • o . , • 4 o . . . . . via the two-photon resonance 2 D5/z state from ground state ~odme and, another via D~/z from spin-orbit excited ~odlne. . . . . . . . 2 o . . . . . . The configuration of core electrons ~s preserved m the ~omzat~on from the Ds/z mtermedmte state, whale the ~omzatlon from the D~/2 state shows two distinct ionization paths, a preserved and a non-preserved path of core electrons.
1. Introduction
Resonance-enhanced multiphoton ionization coupled with photoelectron spectroscopy (REMPI-PES) is a powerful technique to study photoionization dynamics of atomic fragments produced from the photodissociation of molecules [1-8]. Physical quantities concerned in the PES are the kinetic energy and angular distribution o f electrons ejected from neutral states of atomic fragments. The kinetic energy of electron provides the information of internal state of product ion; e.g. the electronically excited state of atomic ion and vibrational state of molecular
* Corresponding author.
ion. The angular distribution of photoelectrons allows the characterization of aligned intermediate states by resonantly absorbed multiphoton and the correlation between the neutral excited state and the ionic state [9-13]. The reactivity of ionic states and the preparation of state-selected ions have received much attention for ion-molecule investigation and its application to plasma and semiconductor process. The conventional PES is based on the kinetic energy measurement of an ejected photoelectron at a fixed polarization angle with respect to the propagation direction of electron. Charged particle imaging techniques coupled with REMPI [14] have the advantage of simultaneous observation and characterization of the angular and kinetic energy distribution of ions and electrons. In this Letter, we present a technique in which the
0009-2614/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0009-2614(95)00880-2
184
W.K. Kang et aL / Chemical Physics Letters 244 (1995) 183-187
three-dimensional spatial distribution of photoelectrons, ejected from iodine atom produced via the photodissociation of the CH3I molecule, has been projected onto two-dimensional surfaces. Atomic iodine, an open-shell atom, has been chosen as a prototype candidate for the production of stateselected ions because of its energy level structure [15]. The ionic state selectivity of iodine atom has been determined through ionization pathways of the intermediate state from the spatial distribution of photoelectrons.
2. Experimental The details of the imaging spectrometer have been described elsewhere for determining the spatial distribution of the iodine atom photofragmented from the CF3I molecule [16]. Photoelectron imaging is essentially the same as that of photoion except that a negative high voltage is applied to the repelling plate. To reduce the effect of ambient magnetic field
on photoelectron trajectories, a ix-metal shield is mounted inside of the time-of-flight tube and the ionization region. A pulsed molecular beam of 3% CH3I seeded in He is injected into the ionization region through a skimmer. Iodine atoms are produced by UV photolysis of CH3I. CH3I dissociates into two channels leading to either a methyl radical and ground state iodine (2P~/z), or a methyl radical and a spin-orbit excited iodine atom (2p?/2). The same laser pulse ionizes the iodine atom via twophoton resonant intermediate state. The UV laser light is produced by doubling the output from a nanosecond Nd:YAG pumped dye laser and is linearly polarized by a half-wave retardation plate. The REMPI spectrum is obtained by monitoring the iodine ion signal and scanning the laser wavelength. The photoelectron cloud expanded from the ionization point is projected onto a microchannel plate/phosphor screen detector at the resonant wavelength of iodine. The images are recorded with an electronic CCD camera (512 × 512 pixels). The CCD camera integrates the images from about 3000 laser
(a)
(b)
0,0 0,2
0,4
0,6
0,8
1.0
Fig. 1. Photoelectron images by (2 + 1) REMPI of: (a) ground state iodine at 304.67 nm and; (b) spin-orbit excited state iodine at 304.02 nm.
W.K. Kang et al. / Chemical Physics Letters 244 (1995) 183-187
shots and the blank image, obtained at non-resonant laser wavelength of iodine atom, is then subtracted.
185
(a) 304.67 nm 1.75 eV
3. Results and discussion
Fig. 1 shows the images of photoelectron ejected from ground state (a) and spin-orbit excited state (b) iodine atoms for alignment of the polarization of laser parallel to detector plate. The ground state iodine atoms are selectively ionized via two-photon o resonance to the (3p) 6p 2 Ds/2 state using 304.67 nm and the spin-orbit excited state iodine via the (3p) 6paD~/2 state using 304.02 nm [15]. Both electron images are anisotropic and depict typical polar-cap appearances. In addition the image from REMPI of excited state iodine atom exhibits an additional inner shell suggesting two ionization paths, i.e. the internal states of iodine ions. The correlation of atomic states in two-photon absorption with linearly polarized light is followed by the symmetry selection rules: AS = 0; A J = 0 , +1, +_2with J=O~--/---~J=l; A L = 0 , _+1, + 2 w i t h L = O ~ / ~ L = l ; a n d A I = 0 , _+2 [7]. While the two-photon transition, (3p) 6p 2D5/2o .__ 2 P~/2 is an allowed transition, the forbidden transition, (3p) 6p4D~/2 ~ ~.plo/e of spin-orbit excited iodine, violating the conservation rule of electron spin, can be described by configuration interaction in the iodine atom. The angular distribution of photoelectrons, obtained by multiphoton ionization utilizing a single linearly polarized laser, is expressed by [9-13] We(0) = Ea2kP2k(cos 0),
(1)
where Pzk(COS 0) is the Legendre polynomial of
order 2k, 0 the angle between light polarization and the photoelectron propagation vector, and azk, the coefficient, contains information on the partial wave character of the outgoing photoelectron in addition to alignment of the resonant intermediate state [12]. For (n + 1) REMPI via the spherically symmetric intermediate state, the expression is reduced to a 0 + a2Pz(cOs 0). The ratio A 2 = ao/a 2 represents the familiar asymmetry parameter /3 used in conventional description of single photon ionization [17]. The asymmetry parameter /3 from inverse Abel transformed image indicates a nearly parallel transi-
(b) 304.02 nm 1.88 eV
~
.
-200
-100
0
100
2;0
Pixel No. Fig. 2. Intensity cross sections through the center of the images: (a) ground state iodine at 304.67 nm and (b) excited state iodine at 304.02 nm.
tion and the intensity pattern with angle 0 shows the polar cap distribution [14]. These are contrasts to previous results that show A 2 coefficients of 1.36 for ionization of the S 4p3p0 state [5] and - 0 . 2 for N o 3p 2 $1/z states [2], respectively. Although no theoretical study has been reported on high-lying state ionization of iodine, the high anisotropy parameter of an electron ejected from the 6p valence shell of iodine is consistent with a maximum value for the electron energy lower than the Cooper minimum as described in the study of /3 for ionization from p subshells of ground-state atoms by Manson [18]. Fig. 2 gives intensity cross sections through the center of images taken at 304.67 nm (a) and 304.02 nm (b) along the direction of laser polarization. The separation of two peaks indicates velocity differences of outgoing electrons from two-photon resonant intermediate states. The velocity of the electron is obtained from v = d/2t where d is the diameter of the electron image and t the time-of-flight of the electron. Since the measurement of t involves large uncertainty, we determine the final state of the product iodine ion by comparing the separation of two electron peaks with the electron energy obtained from energy levels of iodine [15]. The ion states are assigned to the 3P2 state at 304.67 nm, and 3pj
186
W.K. Kang et al. / Chemical Physics Letters 244 (1995) 183-187
120000
1S 0
100000
~Dz
I(11) '7,
IP
_
80000
3p2 4D1/2 (3p)
2D;/z
6O000 304.67 nm
I(I y
~ 304.02 nm
400OO 10000"
2p30/2 (3p)
I(I )
Fig. 3. Schematic diagram depicting relevant energy levels of I(I) and I(II) with (2 + 1) REMPI of iodine atom at 304 nm.
( J = 0 or 1) for the outer-ring and 1D 2 states for the inner-ring at 304.02 nm, respectively. Their separations, 1 : 1.037 : 0.766, agree well with 1 : 1.025 (3P 0) or 1.047 (3P1):0.763 of electron velocity in the literature. The J splitting in the 3Po and 3P1 ionic states is unable to resolve the electron energy gap, 639 cm -1. An energy level diagram of ground and excited state iodines with (2 + 1) REMPI transition is illustrated in Fig. 3. Single-photon ionization involves the removal of a single electron from neutral species without changing the quantum numbers of any other electrons. For an ejected electron with l angular momentum, the change of L between neutral species and the ion is given by AL = 0, + 1. . . . _ I and the change of spin multiplicity becomes AS = _ 1. Since the ionization is achieved by absorbing an additional photon by an intermediate state, we treat these processes as single-photon ionization process thereon. Both ionizations to the 3P2 state from (3p) o state and to 3p0.1 from the (3p) 6p4D~/2 6p 2D5/2 state obey these selection rules and core electrons are preserved. However, the ionization to the ID 2 state o from (3p) 6p 4 D1/2 is a forbidden transition by the above selection rules and core electrons are not preserved. This is in contrast to the ionization of the valence shell np electron (n = 4-6) in excited S and P atoms [5,8]. Their ionizations are driven at ease by preservation of core electron configuration. The 1D 2 ion state selection of iodine may then be followed by the coupling of Rydberg electron and the ion core.
This effect has been found in transition metal atoms [19]. If (3p) 6p4D~/2 state iodine is excited to high-lying Rydberg state by a further photon absorption, 1D 2 state ion can be produced from Rydberg state by rapid radiationless relaxation, autoionization. In summary, photoelectron images of atomic iodine originating from the ground and spin-orbit excited state are first obtained via (2 + 1) REMPI to investigate the angular distribution of the ejected electron and the internal state of product ion in the dissociation of CH3I in the 304 nm region. Both electron images observed from ground and spin-orbit excited iodine atoms have shown cos20 dependence, which is characterized by one-photon ionization from the intermediate state, aligned spherically. The ionization of the (3p) 6p 4 D I / 2 intermediate state by the spin intercombination transition of the 2P~/2 state has the ion-core non preserving ionization path to 1D 2 iodine ion state. This is non-observed transition in p-electron ionization of N, S and P atoms via (2 + 1) REMPI driven by core-electron conservation.
Acknowledgement This work is supported by the Korea Science and Engineering Foundation, which is gratefully acknowledged.
References [1] S.T. Pratt, J.L. Dehmer and P.M. Dehmer, J. Chem. Phys. 67 (1985) 676. [2] S.T. Pratt, J.L. Dehmer and P.M. Dehmer, Phys. Rev. A 36 (1987) 1702. [3] S. Kroll, H. Lundberg, A. Persson and S. Svanberg, Phys. Rev. Letters 55 (1985) 284. [4] W.K. Bischel and L.E, Jusinski, Chem. Phys. Letters 120 (1985) 337. [5] J.R. Appling, M.R. Harbol, R.A. Edington and A.C. Goren, J. Chem. Phys. 97 (1992) 4041. [6] T.V. Venkitachalan and A.S. Rao, Appl. Phys. B 52 (1991) 102. [7] J. Steadman and T. Baer, J. Chem. Phys. 89 (1988) 5507. [8] M.R. Harbol, J.R. Appling and A.C. Goren, J. Chem. Phys. 101 (1994) 2659. [9] P. Lambropoulos, Advan. At. Mol. Phys. 12 (1976) 87. [10] M.P. Strand, J. Hansen, R.-L. Chien and R.S. Berry, Chem. Phys, Letters 59 (1978) 205.
W.K. Kang et al. / Chemical Physics Letters 244 (1995) 183-187
[11] S.N. Dixit and P.L. Lambropoulos, Phys. Rev. A 27 (1983) 168. [12] S.N. Dixit and V. Mckoy, J. Chem. Phys. 82 (1985) 3456. [13] J.R. Appling, M.G. White, W.J. Kessler, R. Fernandez and E.D. Polaikoff, J. Chem. Phys. 88 (1988) 2300. [14] D.W. Chandler and P.L. Houston, J. Chem. Phys. 87 (1987) 1445.
187
[15] C.E. Moore, Atomic energy levels, NSRDS-NBS, Vol. III, Circular No. 35 (US GPO, Washington, 1971) p. 170. [16] Y.S. Kim, W.K. Kang and K.-H. Jung, to be published. [17] J. Cooper and R.N. Zare, J. Chem. Phys. 48 (1968) 942. [18] S.T. Manson, J. Electron Spectry. 1 (1972) 413. [19] L. Sanders, A.D. Sappey and J.C. Weisshaar, J. Chem. Phys. 85 (1986) 6952.