- Email: [email protected]
Rotationally resolved photofragmentation spectroscopy of CH+ formed by resonance-enhanced multiphoton ionization
Volume 185, number 5,6
CHEMICAL PHYSICS LETTERS
25 October 199 I
Rotationally resolved photofragmentation spectroscopy of CH+ formed by resonance-enhanced multiphoton ionization Yumin Wang, Leping Li ’ and W.A. Chupka Sterling Chemislry Laboratory, Yale University, 22s Prospect street, New Haven, CT 06511, KU Received 2 July 199 1; in final form 13 August I99
I
Resonance-enhanced multiphoton dissociation (REMPD) of CH+ yields C+H+ products. By monitoring the H+ signal as a function of either w, (the laser frequency used to carry out REMPI of CH via the E’( 3~0) Rydberg state) or w2 (the frequency used for REMPD of CH+ produced by w,), we are able to determine rotational state distributions of CH+ ions. The observed distributions are in accord with theoretical propensity rules and serve to verify the configuration and symmetry of the Rydberg state. The technique used in this work can serve as an alternative to laser-induced fluorescence when the latter is impractical.
1. Introduction
The investigation of photoionization processes of molecules has attracted much attention in the past several decades. However, the traditional one-photon photoionization studies suffered from the inability to determine the rotational distribution of the ions because of the inability of conventional photoelectron spectroscopy to resolve rotational structure except in a very few favorable cases [ l,? 1. In recent years, with the availability of intense tunable lasers, resonance-enhanced multiphoton ionization (REMPI) studies have become a powerful tool in studying these processes. In (n t 1) REMPI studies a specific rovibrational level of a highly excited electronic state is selected by the n-photon resonance process, one more photon is absorbed by the excited state and specific rovibrational levels of the ion are thus produced. New techniques have been employed in recent years to detect the ions produced in the REMPI processes. REMPI PES studies are particularly useful in studying molecules with large rotational constants and thus large rotational energy spacings, the H2 molecule for example [3,4]. For other molecules, because of insufficient resolution, REMPI PES usu’ Permanent address: IBM Co., East Fishkill, Rte. 52, Hopewell Junctton, New York, NY 12533, USA.
478
ally cannot resolve the rotational structure except in a very few cases. One such exception is the REMPl PES of NO. The final rotational distribution of the NO+ ion was studied by measuring zero kinetic energy (ZEKE) photoelectrons [5,6] and in other studies by recording the time-of-flight distribution of photoelectrons arising from the photoionization of high J levels [ 7,8]. Laser-induced fluorescence studies of ions produced in a REMPI process have been used in some diatomic molecules [ 9,101. In this paper, we report another way to determine the rotational distribution of ions produced by REMPl namely rotationally resolved photofragmentation, using CH as an example.
2. Experimental CH radicals were generated by photodissociation of bromoform (CHBrJ) at 266 nm (0,). Bromoform of vapor pressure at room temperature was directed into the ionization region of a time-of-flight mass spectrometer and photodissociated into CH and other products. CH thus produced was then resonantly ionized by a focused laser beam from a Nd: YAG-pumped dye laser (w, ). CH+ was probed by a loosely focused laser beam from an excimerpumped dye laser (w,) and then photofragmented by w,. Elsevier Science Publishers B.V.
Volume 185, number 5,6
CHEMICALPHYSICSLETTERS
The photolysis laser beam ( w3) is the fourth harmonic of a Nd:YAG DCR-11. The dye laser (w,) pumped by the Nd: YAG DCR-2A was a Quanta-Ray PDL-1 operated with DCM (310-320 nm). The dye laser ( 02) pumped by the XeCl excimer (Questek 2240) was Lumonics Hyperdye-300 operated with stilbene 420. The ions produced were amplified and collected by a Tektronix 7912D programmable digitizer interfaced with an LSI 11/23 computer for mass analysis. Wavelength scans were carried out by monitoring the m/e=13 (CH+), 12 (C’) and 1 ( HC ) ion mass channels. All time sequences were connected by a Stanford Research System DG535 delay/pulse generator interfaced to an LSI 11/23 computer. The delay/pulse generator was triggered by the oscillator output of the Nd:YAG. At 3.2 ms delay, the Q-switch of the Nd: YAG and the excimer laser were fired by the delay/pulse generator. In order to minimize the effect of CH+ ions produced non-resonantly by photolysis laser alone, a 200 ps delay was used between the photolysis laser and the other two lasers.
3. Results and discussion
25 October 1991
l(a). CH+ ion channel
L
63600
’
I
63400
I
63200
I
63
30
Two-photon Energy (cm.‘)
The time overlap between w, and w2 in this experiment was very crucial since H’ ions were seen only when the two beams were well overlapped in time. Fig. 1 shows the potential energy curves of some of the states of CH+. In order to produce C+H+ photodissociation products, one more photon from w, must be absorbed from the A ‘n state of CH+. The whole process can be described as follows: ( 1) CH is produced by photolysis of CHBr3 with 266 nm (w,) light: nw, CHBr, --+ CH t other products . (2 ) CH+ (X ‘II, zi=O, N ’ ) is formed by two-phoionization via the ton resonant, one-photon (3po)E’(*Z+, U’~0: N’ ) level by w, in the wavelength region between 3 14 and 3 17 nm. The E’ Rydberg state spectrum is displayed by monitoring the CH+ ion mass channel:
Fig. I. Potential energy curves of CH+ ion and the illustration of bound-free and bound-bound-free transitions which give rise to C+ and H+ ion mass channels respectively. 2Wl
CH(XZII,v=O,N”---CH*(E’*C+U’=O,N’) (UI -CH+(X’C+,v+=O,N+). (3) The rotational population of the CH + is monitored by a one-photon resonance-enhanced photofragmentation process which produces a 3-color dependent H+ ion signal: CH+(XlE+,v+=O,N+)%H+(A’l-I,v=O,N) :
CtH+.
Examination of fig. 1 shows that a single photon of frequency o2 does not have enough energy to reach the dissociation continuum from the A state of CH+ to give C + H+ dissociation products. Thus when w, 479
comes earlier than w2, even though wZ still carries out the A ‘lI( v=O)+X ‘Et (v=O) transition, w, is ahead in time so that it could not photodissociate CH+ in the A ‘II(u=O, N) rovibronic state populated by oz. When wI comes later than w2, there is just simply no population in the X ‘C+ (u+ =0) state for w2 to make the A’II( v=O)+X ‘C+(v=O) transition and thus no H + ion could be produced. Therefore, only when o, and o2 are overlapped in time could the H’ ion be produced. Fig. 2a shows the 0 branch of a (2 t 1) REMPI spectrum of the (3po)E’(‘C+, v=0)+X2FI(v=O) transition as monitored using the CH+ ion intensity. A simultaneous recording of the C+ ion channel shows the same structure as that of the CHS ion channel which indicates that some CH+ ions absorb another photon to give the dissociation products C+ + H (fig. 2b) but no significant amount of Hf. The H + ion is shown to be three laser dependent by the observation that blocking any one of the three lasers eliminates the H+ signal completely. Fig. 2c shows the H+ signal recorded by fixing the o2 laser
30, /
25 October 1991
CHEMICALPHYSICSLETTERS
Volume 185,number 5,6
,&
CO +H+ COP) + H’
.-
at the Q(2) line of the CH+ A’IT(u=O)+X’C+ (Y=O) transition and scanning the w, laser. Examination of fig. 2c shows a drastic reduction in intensity of all lines except the O(3), O(5) and O(7) lines. Since wZ is fixed at the Q(2) line of the A ‘Il(u=O)+X ‘Z+(v=O) transition, only excitarotational levels of the of those tion (3po)E’ ‘C+( v=O) Rydberg state of CH which can be ionized to populate the N + = 2 rotational level of the CH+ X ‘C+ (u= 0) state give rise to H+ signal by resonance-enhanced photodissociation of CH+. In a two-photon transition, the selection rule for the 0 branch is N’ = N” -2, where N’ and N” represent the rotational quantum numbers of the upper and lower states of the transition, thus N’ is equal to I, 3 and 5 for O(3), O(5) and O(7) lines respectively. Therefore, only the N’ = 1, 3 and 5 rotational levels of the (3po)E’ ‘Cf (v=O) state are ionized to populate the N +=2 level of the ion. A Herzberg diagram indicating the relevant transitions between the (3po)E’ ‘C+(v=O) state of CH and the X ‘C+ (v=O) state of CH+ ion is presented in fig. 3. The intermediate state (3po)E’ ‘Et of CH and the X ‘C+ state of CH+ both belong to Hund’s case (b) coupling. For the final continuum state (ion plus photoelectron) N +, the total angular momenturn exclusive of spin of the system CH+ (‘E+) Se(U), is given by 0 CH+X’.l+
+ks
0
N+=O
1
N+=o
1
3
+X’P+kd
1
2
j
4
N+=O
5
6
4
5
6
J
\
0
4
0 1 12.3
5
Internuclear Distance ( 8, ) Fig. 2. Photoionization spectrum for w2fixed at Q (2) line of the A’n(u=O)-X ‘Z+(v=O) transltion ofCH+ while w, is scanned by monitoring (a) CH+ ion mass channel, (b) C+ ion mass channel, (c) H+ ion mass channel.
480
(~P~)E’~~N~=O
1
2
3
Fig. 3. Schematic diagram of the allowed rotational levels of the X ‘I+ (v=O) state of the ion produced by one-photon ionization process from the (3pu)E’ *Z+(v= 0) Rydberg state of CH.
Volume 185, number 5,6
N+=N+tl,
N+-N’=&l,
N+=N+tl,N+tl-I,...,
IN+-11 ,
(1)
where 1is the partial wave of the photoelectron. The ionization is a one-photon process, and the selection rule for direct one-photon ionization in the electric dipole approximation is: N+-N’=O,
-tl
(2)
and the parity selection rule is (+)-(-).
(3)
For the E’ 2z+ Rydberg state of CH with Hund’s case (b ) coupling, the overall parity is ( - I)“‘. The parity of the X ‘C’ state of CH+ is (- 1)N+ and the parity of the photoelectron is ( - I )‘. Thus the overall parity for the ion plus electron is ( - 1) N“! Since the intermediate state is a 3~0 Rydberg state, we expect the I=0 and 2 partial waves to predominate and therefore the total parity ofthe continuum is the same as that of the ion. From the Herzberg diagram in fig. 3 we can conclude that the selection rule (in our case) is N+-N’=?l,
?3.
N + -N’ +/t-p, +p+ =odd,
(5)
wherep, andp, denote the Kronig symmetry indices for the ionic and excited states respectively. These indices are 0 for e-type states (C’, II+, .. ) and are 1 for f-type states (I-, n-, ...). In our case, both Rydberg state and ionic state are e-type states (C+ :j, therefore the relation can be reduced to N + -AT’ t != odd. More specifically,
N+-N’=O,
* 1, k2,
+3,
foradpartialwave.
We note that for homonuclear diatomic molecules with rigorous u-g symmetry, A/=odd only. For a Rydberg state of a heteronuclear molecule, such as CH, this rule holds approximately. Fig. 2c shows that for N + = 2 there is a strong signal in the H+ channel for N’ = 1, 3 and 5 in accord with theory since we expect the dominant partial waves to be s and d from the (3po)E’ ‘I;+ Rydberg state. A much weaker signal is seen for N’ = 2 and 4 due to a weaker p partial wave. Thus our observation is in good agreement with the photoionization propensity rule given above.
4. Conclusion Resonance-enhanced multiphoton dissociation is used to determine the rotational distribution of CH’ ions produced by single photon ionization of selected rotational levels of the (3po)E’ *C+ state of CH. The observed distribution is in accord with theoretical predictions. The technique described here offers an alternative method to laser-induced fluorescence with spectroscopic resolution.
(4)
The key relationships between N + and N’ have been established by Dixit and McKay [ 111 and by Xie and Zare [ 121 theoretically. In general, if the resonant intermediate state and the ionic state can both be represented in Hund’s case (b), the allowed values of the ionic rotational quantum number N + are related to the intermediate state rotational quantum number N’ and the partial wave I of the outgoing electron through the relation:
N+-N’=
25 October 199 I
CHEMICAL PHYSICS LETTERS
for an s partial wave, for a p partial wave ,
Acknowledgement The authors wish to acknowledge helpful discussion with Professor S.D. Colson. This work was supported by the National Science Foundation (CHE8821032).
References [ I ] D.W. Turner, A.D. Baker, C. Baker and C.R. Brundle, Molecular photoelectron spectroscopy, a handbook of He 584 A spectra (Interscience, New York, 1970). [21 J.H.D. Eland, Photoelectron spectroscopy (Wiley, New York 1974). [31 S.T. Pratt, P.M. Dehmer and J.L. Dehmer, J. Chem. Phys. 78 ( 1983) 4315. [41 M.A. O’Halloran, S.T. Pratt, P.M. Dehmerand J.L. Dehmer, J. Chem. Phys. X7 (1987) 3288. [ 51 K. Miiller-Detlefs, M. Sander and E.W. Schlag, Chem. Phys. Letters 112 (1987) 291. [ 61 M. Sander, L.A. Chewter, K. Miiller-Detlefs and E.W. Schlag, Phys. Rev. A 36 (1987) 4543.
481
Volume 185, number 5,6
CHEMICAL PHYSICS LETTERS
[7 ] KS. Viswanathan, E. Sekreta, E.R. Davidson and J.P. Reilly. J. Phys. Chem. 90 (1986) 5978. [S] SW. Allendorf, D.J. Leahy, DC. Jacobs and R.N. Zare, J. Chem. Phys. 91 (1989) 2216. [9] A. Fujii, T. Ebata and M. Ito, J. Chem. Phys. 88 (1988) 5307; 161 (1989) 93.
482
25 October 1991
[ IO] J. Xie and R.N. Zare, Chem. Phys. Letters 159 (1989) 399. [ I 1 ] S.N. Dixit and V. McKay, Chem. Phys. Letters 128 ( 1986) 49. [ 121 J. Xie and R.N. Zare, J. Chem. Phys. 93 (1990) 3033.
CHEMICAL PHYSICS LETTERS
25 October 199 I
Rotationally resolved photofragmentation spectroscopy of CH+ formed by resonance-enhanced multiphoton ionization Yumin Wang, Leping Li ’ and W.A. Chupka Sterling Chemislry Laboratory, Yale University, 22s Prospect street, New Haven, CT 06511, KU Received 2 July 199 1; in final form 13 August I99
I
Resonance-enhanced multiphoton dissociation (REMPD) of CH+ yields C+H+ products. By monitoring the H+ signal as a function of either w, (the laser frequency used to carry out REMPI of CH via the E’( 3~0) Rydberg state) or w2 (the frequency used for REMPD of CH+ produced by w,), we are able to determine rotational state distributions of CH+ ions. The observed distributions are in accord with theoretical propensity rules and serve to verify the configuration and symmetry of the Rydberg state. The technique used in this work can serve as an alternative to laser-induced fluorescence when the latter is impractical.
1. Introduction
The investigation of photoionization processes of molecules has attracted much attention in the past several decades. However, the traditional one-photon photoionization studies suffered from the inability to determine the rotational distribution of the ions because of the inability of conventional photoelectron spectroscopy to resolve rotational structure except in a very few favorable cases [ l,? 1. In recent years, with the availability of intense tunable lasers, resonance-enhanced multiphoton ionization (REMPI) studies have become a powerful tool in studying these processes. In (n t 1) REMPI studies a specific rovibrational level of a highly excited electronic state is selected by the n-photon resonance process, one more photon is absorbed by the excited state and specific rovibrational levels of the ion are thus produced. New techniques have been employed in recent years to detect the ions produced in the REMPI processes. REMPI PES studies are particularly useful in studying molecules with large rotational constants and thus large rotational energy spacings, the H2 molecule for example [3,4]. For other molecules, because of insufficient resolution, REMPI PES usu’ Permanent address: IBM Co., East Fishkill, Rte. 52, Hopewell Junctton, New York, NY 12533, USA.
478
ally cannot resolve the rotational structure except in a very few cases. One such exception is the REMPl PES of NO. The final rotational distribution of the NO+ ion was studied by measuring zero kinetic energy (ZEKE) photoelectrons [5,6] and in other studies by recording the time-of-flight distribution of photoelectrons arising from the photoionization of high J levels [ 7,8]. Laser-induced fluorescence studies of ions produced in a REMPI process have been used in some diatomic molecules [ 9,101. In this paper, we report another way to determine the rotational distribution of ions produced by REMPl namely rotationally resolved photofragmentation, using CH as an example.
2. Experimental CH radicals were generated by photodissociation of bromoform (CHBrJ) at 266 nm (0,). Bromoform of vapor pressure at room temperature was directed into the ionization region of a time-of-flight mass spectrometer and photodissociated into CH and other products. CH thus produced was then resonantly ionized by a focused laser beam from a Nd: YAG-pumped dye laser (w, ). CH+ was probed by a loosely focused laser beam from an excimerpumped dye laser (w,) and then photofragmented by w,. Elsevier Science Publishers B.V.
Volume 185, number 5,6
CHEMICALPHYSICSLETTERS
The photolysis laser beam ( w3) is the fourth harmonic of a Nd:YAG DCR-11. The dye laser (w,) pumped by the Nd: YAG DCR-2A was a Quanta-Ray PDL-1 operated with DCM (310-320 nm). The dye laser ( 02) pumped by the XeCl excimer (Questek 2240) was Lumonics Hyperdye-300 operated with stilbene 420. The ions produced were amplified and collected by a Tektronix 7912D programmable digitizer interfaced with an LSI 11/23 computer for mass analysis. Wavelength scans were carried out by monitoring the m/e=13 (CH+), 12 (C’) and 1 ( HC ) ion mass channels. All time sequences were connected by a Stanford Research System DG535 delay/pulse generator interfaced to an LSI 11/23 computer. The delay/pulse generator was triggered by the oscillator output of the Nd:YAG. At 3.2 ms delay, the Q-switch of the Nd: YAG and the excimer laser were fired by the delay/pulse generator. In order to minimize the effect of CH+ ions produced non-resonantly by photolysis laser alone, a 200 ps delay was used between the photolysis laser and the other two lasers.
3. Results and discussion
25 October 1991
l(a). CH+ ion channel
L
63600
’
I
63400
I
63200
I
63
30
Two-photon Energy (cm.‘)
The time overlap between w, and w2 in this experiment was very crucial since H’ ions were seen only when the two beams were well overlapped in time. Fig. 1 shows the potential energy curves of some of the states of CH+. In order to produce C+H+ photodissociation products, one more photon from w, must be absorbed from the A ‘n state of CH+. The whole process can be described as follows: ( 1) CH is produced by photolysis of CHBr3 with 266 nm (w,) light: nw, CHBr, --+ CH t other products . (2 ) CH+ (X ‘II, zi=O, N ’ ) is formed by two-phoionization via the ton resonant, one-photon (3po)E’(*Z+, U’~0: N’ ) level by w, in the wavelength region between 3 14 and 3 17 nm. The E’ Rydberg state spectrum is displayed by monitoring the CH+ ion mass channel:
Fig. I. Potential energy curves of CH+ ion and the illustration of bound-free and bound-bound-free transitions which give rise to C+ and H+ ion mass channels respectively. 2Wl
CH(XZII,v=O,N”---CH*(E’*C+U’=O,N’) (UI -CH+(X’C+,v+=O,N+). (3) The rotational population of the CH + is monitored by a one-photon resonance-enhanced photofragmentation process which produces a 3-color dependent H+ ion signal: CH+(XlE+,v+=O,N+)%H+(A’l-I,v=O,N) :
CtH+.
Examination of fig. 1 shows that a single photon of frequency o2 does not have enough energy to reach the dissociation continuum from the A state of CH+ to give C + H+ dissociation products. Thus when w, 479
comes earlier than w2, even though wZ still carries out the A ‘lI( v=O)+X ‘Et (v=O) transition, w, is ahead in time so that it could not photodissociate CH+ in the A ‘II(u=O, N) rovibronic state populated by oz. When wI comes later than w2, there is just simply no population in the X ‘C+ (u+ =0) state for w2 to make the A’II( v=O)+X ‘C+(v=O) transition and thus no H + ion could be produced. Therefore, only when o, and o2 are overlapped in time could the H’ ion be produced. Fig. 2a shows the 0 branch of a (2 t 1) REMPI spectrum of the (3po)E’(‘C+, v=0)+X2FI(v=O) transition as monitored using the CH+ ion intensity. A simultaneous recording of the C+ ion channel shows the same structure as that of the CHS ion channel which indicates that some CH+ ions absorb another photon to give the dissociation products C+ + H (fig. 2b) but no significant amount of Hf. The H + ion is shown to be three laser dependent by the observation that blocking any one of the three lasers eliminates the H+ signal completely. Fig. 2c shows the H+ signal recorded by fixing the o2 laser
30, /
25 October 1991
CHEMICALPHYSICSLETTERS
Volume 185,number 5,6
,&
CO +H+ COP) + H’
.-
at the Q(2) line of the CH+ A’IT(u=O)+X’C+ (Y=O) transition and scanning the w, laser. Examination of fig. 2c shows a drastic reduction in intensity of all lines except the O(3), O(5) and O(7) lines. Since wZ is fixed at the Q(2) line of the A ‘Il(u=O)+X ‘Z+(v=O) transition, only excitarotational levels of the of those tion (3po)E’ ‘C+( v=O) Rydberg state of CH which can be ionized to populate the N + = 2 rotational level of the CH+ X ‘C+ (u= 0) state give rise to H+ signal by resonance-enhanced photodissociation of CH+. In a two-photon transition, the selection rule for the 0 branch is N’ = N” -2, where N’ and N” represent the rotational quantum numbers of the upper and lower states of the transition, thus N’ is equal to I, 3 and 5 for O(3), O(5) and O(7) lines respectively. Therefore, only the N’ = 1, 3 and 5 rotational levels of the (3po)E’ ‘Cf (v=O) state are ionized to populate the N +=2 level of the ion. A Herzberg diagram indicating the relevant transitions between the (3po)E’ ‘C+(v=O) state of CH and the X ‘C+ (v=O) state of CH+ ion is presented in fig. 3. The intermediate state (3po)E’ ‘Et of CH and the X ‘C+ state of CH+ both belong to Hund’s case (b) coupling. For the final continuum state (ion plus photoelectron) N +, the total angular momenturn exclusive of spin of the system CH+ (‘E+) Se(U), is given by 0 CH+X’.l+
+ks
0
N+=O
1
N+=o
1
3
+X’P+kd
1
2
j
4
N+=O
5
6
4
5
6
J
\
0
4
0 1 12.3
5
Internuclear Distance ( 8, ) Fig. 2. Photoionization spectrum for w2fixed at Q (2) line of the A’n(u=O)-X ‘Z+(v=O) transltion ofCH+ while w, is scanned by monitoring (a) CH+ ion mass channel, (b) C+ ion mass channel, (c) H+ ion mass channel.
480
(~P~)E’~~N~=O
1
2
3
Fig. 3. Schematic diagram of the allowed rotational levels of the X ‘I+ (v=O) state of the ion produced by one-photon ionization process from the (3pu)E’ *Z+(v= 0) Rydberg state of CH.
Volume 185, number 5,6
N+=N+tl,
N+-N’=&l,
N+=N+tl,N+tl-I,...,
IN+-11 ,
(1)
where 1is the partial wave of the photoelectron. The ionization is a one-photon process, and the selection rule for direct one-photon ionization in the electric dipole approximation is: N+-N’=O,
-tl
(2)
and the parity selection rule is (+)-(-).
(3)
For the E’ 2z+ Rydberg state of CH with Hund’s case (b ) coupling, the overall parity is ( - I)“‘. The parity of the X ‘C’ state of CH+ is (- 1)N+ and the parity of the photoelectron is ( - I )‘. Thus the overall parity for the ion plus electron is ( - 1) N“! Since the intermediate state is a 3~0 Rydberg state, we expect the I=0 and 2 partial waves to predominate and therefore the total parity ofthe continuum is the same as that of the ion. From the Herzberg diagram in fig. 3 we can conclude that the selection rule (in our case) is N+-N’=?l,
?3.
N + -N’ +/t-p, +p+ =odd,
(5)
wherep, andp, denote the Kronig symmetry indices for the ionic and excited states respectively. These indices are 0 for e-type states (C’, II+, .. ) and are 1 for f-type states (I-, n-, ...). In our case, both Rydberg state and ionic state are e-type states (C+ :j, therefore the relation can be reduced to N + -AT’ t != odd. More specifically,
N+-N’=O,
* 1, k2,
+3,
foradpartialwave.
We note that for homonuclear diatomic molecules with rigorous u-g symmetry, A/=odd only. For a Rydberg state of a heteronuclear molecule, such as CH, this rule holds approximately. Fig. 2c shows that for N + = 2 there is a strong signal in the H+ channel for N’ = 1, 3 and 5 in accord with theory since we expect the dominant partial waves to be s and d from the (3po)E’ ‘I;+ Rydberg state. A much weaker signal is seen for N’ = 2 and 4 due to a weaker p partial wave. Thus our observation is in good agreement with the photoionization propensity rule given above.
4. Conclusion Resonance-enhanced multiphoton dissociation is used to determine the rotational distribution of CH’ ions produced by single photon ionization of selected rotational levels of the (3po)E’ *C+ state of CH. The observed distribution is in accord with theoretical predictions. The technique described here offers an alternative method to laser-induced fluorescence with spectroscopic resolution.
(4)
The key relationships between N + and N’ have been established by Dixit and McKay [ 111 and by Xie and Zare [ 121 theoretically. In general, if the resonant intermediate state and the ionic state can both be represented in Hund’s case (b), the allowed values of the ionic rotational quantum number N + are related to the intermediate state rotational quantum number N’ and the partial wave I of the outgoing electron through the relation:
N+-N’=
25 October 199 I
CHEMICAL PHYSICS LETTERS
for an s partial wave, for a p partial wave ,
Acknowledgement The authors wish to acknowledge helpful discussion with Professor S.D. Colson. This work was supported by the National Science Foundation (CHE8821032).
References [ I ] D.W. Turner, A.D. Baker, C. Baker and C.R. Brundle, Molecular photoelectron spectroscopy, a handbook of He 584 A spectra (Interscience, New York, 1970). [21 J.H.D. Eland, Photoelectron spectroscopy (Wiley, New York 1974). [31 S.T. Pratt, P.M. Dehmer and J.L. Dehmer, J. Chem. Phys. 78 ( 1983) 4315. [41 M.A. O’Halloran, S.T. Pratt, P.M. Dehmerand J.L. Dehmer, J. Chem. Phys. X7 (1987) 3288. [ 51 K. Miiller-Detlefs, M. Sander and E.W. Schlag, Chem. Phys. Letters 112 (1987) 291. [ 61 M. Sander, L.A. Chewter, K. Miiller-Detlefs and E.W. Schlag, Phys. Rev. A 36 (1987) 4543.
481
Volume 185, number 5,6
CHEMICAL PHYSICS LETTERS
[7 ] KS. Viswanathan, E. Sekreta, E.R. Davidson and J.P. Reilly. J. Phys. Chem. 90 (1986) 5978. [S] SW. Allendorf, D.J. Leahy, DC. Jacobs and R.N. Zare, J. Chem. Phys. 91 (1989) 2216. [9] A. Fujii, T. Ebata and M. Ito, J. Chem. Phys. 88 (1988) 5307; 161 (1989) 93.
482
25 October 1991
[ IO] J. Xie and R.N. Zare, Chem. Phys. Letters 159 (1989) 399. [ I 1 ] S.N. Dixit and V. McKay, Chem. Phys. Letters 128 ( 1986) 49. [ 121 J. Xie and R.N. Zare, J. Chem. Phys. 93 (1990) 3033.