- Email: [email protected]
(2 + 1) resonance-enhanced multiphoton ionization studies of the CH D 2Π (ν=2) state
Volume 192, number
4
CHEMICAL
PHYSICS
LETTERS
8 May 1992
(2 + 1) resonance-enhanced multiphoton ionization studies of the CH D 213(v= 2 ) state Yumin
Wang,
Leping
Li ’ and William
A. Chupka
Sterling Chemistry Laboratory, Yale University, 225 Prospect Street, New Haven, CT 065 II, USA Received
2 I January
1992
(2 + 1) resonance-enhanced multiphoton ionization studies are performed on the D *H( v= 2) state of CH. Anomalous line intensities are shown to be due to an accidental near resonance with the C %+ state at the one-photon level. The sudden onset of predissociation for Na I2 of this level is investigated and is attributed to complex interactions among three *IT states in this energy
region.
1. Introduction In a vacuum UV study of CH, Herzberg and Johns [ I ] observed transitions to an nd Rydberg series. In addition, transitions terminating in three other states, designated D ‘I-Ii, E *II, and F 2Z+ respectively, were also observed. The D state was assigned a configuration of 2pn3 based on the sign and magnitude of its spin-orbit coupling constant. The F state was assigned to the 3po Rydberg configuration. However, the E state was left unassigned although several possible candidates were discussed. Later in a (2 + 1) REMPI study of CH, Chen et al. [ 2-41 re-examined this spectral region. Aided by the photoelectron spectrum they assigned the E state as the v=2 vibrational level of the D state and suggested that it was predissociated in some undetermined manner. The F state was not observed but instead several spectral features appeared in a longer wavelength region than the F state and were assigned to the 3po Rydberg configuration based on the appearance of the photoelectron spectrum. The latter discrepancy remains to be resolved and will not be discussed further in this publication. Recently Tjossem and Smyth [ 51 observed the MPI spectrum of the D state v=2 level under very r Present address: IBM Co., East Fishkill, Junction, New York, NY 12533, USA.
348
Rte. 52, Hopewell
0009-2614/92/$
different experimental conditions and found dramatically different relative line intensities. They suggested that these differences were caused by saturation due to a near accidental resonance at the onephoton level. Recent theoretical calculations [6] predict that three *lI states lie in the energy region of the D *H (v= 2) state. Thus complex interactions among these II states are expected and would influence the appearance of the MPI spectrum. In addition, the C state, which is a low lying valence state, falls in the one-photon energy region and therefore the two-photon transition strength to the D (u= 2 ) state will be further influenced by the C state. The purpose of this work is to verify the suggestion of Tjossem and Smyth [ 51 that the near resonance of the C state at the one-photon level influences the total (2 + 1) MPI intensity and to explain the predissociation of the D state Y= 2 level using recent calculations [ 61 of excited states of CH.
2. Experimental The experimental procedure has been described elsewhere [ 7]_ CH radicals were formed by photodissociation of bromoform with 266 nm light. CH thus produced was then probed by a frequency doubled, Nd: YAG pumped dye laser. The dye laser was operated with DCM for the spectral region of inter05.00 0 1992 Elsevier Science Publishers
B.V. All rights reserved.
Volume 192, number 4
8 May 1992
CHEMICAL PHYSICS LETTERS
est. The ion signal produced by resonance enhanced ionization of the probe laser was then amplified and collected by a Tektronix 79 12D programmable digitizer interfaced with an LSI 1 l/23 computer for mass analysis. Wavelength scans were carried out while monitoring the m/e= 13 (CH+ ) ion mass channel. All time sequences were connected by a Stanford Research System DG535 delay/pulse generator interfaced to an LSI 11/23 computer.
Q(s)
(4
I
Q(9)
Q(l'J)
Q(‘)
3. Results and discussion The D *II( u= 2) state has been observed by Chen et al. [ 21 in a (2 + 1) REMPI study of CH photodissociated from ketene in the same laser pulse. Their spectra show smooth rotational envelopes and complete 0, P, R, and S branches. In contrast to these observations, a (2 + 1) REMPI study of this state by Tjossem and Smyth [ 5 ] in a flame environment using weak laser intensity shows very different rotational line intensities. In their spectrum, only two pair of lines corresponding to the Q( 8) and Q( 9) transitions in the Q branch and several weak lines in the R branch were observed. More lines appeared when the laser intensity was increased, but the overall signal to noise ratio degraded due to increased background ionization. Noticing that the C 2C+ (v= 0) level lies close to the one-photon energy, they concluded that the lines they observed were enhanced by an accidental double resonance with the C state as the intermediate state. However, it is not clear from a comparison of the data whether the disagreement between the two sets of experiments is due to the power broadening of the C state or some other factors. Therefore, we conducted a series of experiments on the Q and R branches of the D( u= 2) t +X (v= 0) two-photon transition using different laser powers. Figs. la-lc show the spectra obtained using different laser intensities. The laser intensity was increased gradually from la to lc. It is immediately apparent that the relative intensities of the rotational lines change with laser intensity. Comparison was also made with the spectra of Tjossem et al. The two components Q1 and Q2 of the Q( 8) line which were well resolved in their spectra, were only partially resolved in fig. la which implies that our laser inten-
(cl
, ,, 4 bL__-__ 64000
64100
Two-photon
Energy
63900
(UT-~)
Fig. 1. Rotational intensity distributions of the Q branch of the DZI-I(u=2) + +X *II ( ZJ=0) two-photon transition as a function of the laser intensity. The laser intensity is increased gradually from (a) to (c).
sity was greater than theirs. Upon going from la to 1b to lc, the Q, (8) and Q2 ( 8 ) lines change from partially resolved to barely resolved to completely unresolved. This is a very strong sign of saturation either of the transition or of the signal detection or both. Thus upon increasing laser intensity, the Q ( 8 ) lines were saturated and reduced in intensity relative to other lines. Finally at very high laser intensity the spectral appearance approaches that of Chen et al. [ 21. This explanation is therefore in accordance with the tentative suggestion given by Tjossem and Smyth [ 5 ] for the differences between their spectrum and that of Chen et al. [2]. We also concur with the qualitative explanation given by Tjossem and Smyth for the extraordinarily enhanced intensity of the Q(8) line in their spectrum as being due to a near accidental coincidence 349
Volume 192, number
CHEMICAL
4
PHYSICS
(A E 7 cm-’ ) of the first photon with the R ( 8 ) line of the CtX transition followed by P( 9) of the D-C band. Fig. 2 illustrates a construction which is very convenient for a qualitative consideration of such near resonance cases [ 81. In the figure are plotted the energies of the R,, Q, and Pi branches of the C *x+ (v= 0) +-X *II( v= 0) transitions as well as the Rz, Q2 and P2 branches of the D *II( v=2)c C *C+ (v=O) transition. A schematic diagram showing those relevant transitions is presented in the lower left corner. The intervals between the upper and lower curves, e.g. between R, and P, for every N are equal to twice the values of A defined as the detunings between the intermediate state rotational energy and the one-photon energy. The relative intensities of the overall two-photon transitions thus can be inferred qualitatively from this diagram. For example, RI +P2 corresponds to the Q branch of the two-photon DttX transition, while the Q branch can also be reached via the Q1 +Q2 path. The contribution of the two paths to the Q branch intensity can be seen in the figure. The R, and P2 curves cross between N= 9 and 10 and the values of A are 7 and 34 cm-’ respectively. From the insert at the lower left comer, one can see that the path R, (N- 1) + P2 (N) corresponds to the Q (N- 1) line in the two-photon tran-
8 May 1992
LETTERS
sition. Therefore, the Q (8) and Q( 9) lines are enhanced, with Q (8) having more enhancement since it has a smaller value of A. Thus this explains the intensity of the Q branch under saturation-free condition. Experiments were also performed on the P branch ofthe two-photon D *II( ~=2)ttX *II(v=O) transition and the results are shown in fig. 3 where the spectrum of fig. 3b was taken at higher laser power than that of fig. 3a. Notice that those lines marked with an asterisk were assigned to the R branch of the E’*C+(V=O)++X*Z+(V=O) two-photon transition. The line marked with an arrow is due to the mass- 12 carbon ion contamination due to Coulomb broadening of the mass spectrum. Examination of fig. 2 shows that the Q, +P2 path is the dominant one for the P branch of the two-photon transition and the two curves cross between N= 13 and 14. Therefore, the P( 13 ) and P ( 14) lines should be enhanced by the intermediate C state. However, even at relatively low laser power, the P( 13) line intensity accounts for approximately only 20% of that of the P ( 12) line which is much less enhanced by the intermediate state. Therefore, there must be other decay channels which compete with the ionization.
(a)
I
IJ c
PilO)
2
_c
s
P(B)
p(g)
x
x
I
I
63900 5
10 Rotational
15 Quantum
20 Number
25
I
I
63700
63600
30
(N)
Fig. 2. Schematic diagram to predict the enhancement of the D211(v=2) t +X lII( v=O) two-photon transition by the nearly resonant intermediate C *Z+ (u= 0) state at the one-photon level. The insert at the lower left comer shows the relevant branches for each one photon transition. The curves represent the relevant transition energies of the branches as a function of the rotational quantum number N of the intermediate C state.
350
I
63800 Two-photon
Energy ( cm-l )
Fig. 3. Rotational intensity distributions of the P branch of the D211(v=2) ++X *II(v=O) two-photon transition as a function of the laser intensity. The spectrum shown in (b) was taken at higher laser intensity than that shown in (a). The lines marked with an asterisk were due to other transitions. The line marked with an arrow is due to the mass- 12 carbon ion contamination resulting from the Coulomb broadening.
Volume 192, number
4
CHEMICAL
PHYSICS
Furthermore the decay rate must vary greatly for different rotational levels of the D ( u= 2) state. The theoretical formalism for two-photon transition line strengths with one nearly resonant state at the one-photon level has been given by Mainos et al. [ 91. The calculation for the two-photon transition moment involves explicit considerations of complicated angular momentum couplings and is rather involved. However, the dominant factor in the calculation is the energy detuning between the onephoton energy and the energy of the intermediate state. Therefore a rough estimation can be made for the two-photon transition. For the P branch of the D(v=2) t +-X (v= 0) two-photon transition, as discussed above, the P( 13) and P( 14) should be enhanced. The ionization intensity is a function of the rotational temperature of the ground state as well as of the rotational line strength. It also depends on competing decays such as predissociation. The rotational temperature of CH formed from the photolysis of bromoform has been estimated to be roughly 2500 K [ lo]. The line strength for P( 13) can be estimated, by using only the energy detuning of the one-photon energy and the Boltzmann factor at this temperature, to be roughly five times stronger than that of the P( 12) if population decays of the upper levels are not considered. The sudden decrease in intensity of the P (N ) with N> 13 cannot be explained by a typical heterogeneous predissociation. In their VUV studies, Herzberg and Johns [ 1 ] observed that the D( v= 2 ) +X (EtX in their notation) band shows a slight diffuseness which did not vary with J from which they concluded that the E ‘II is subject to a homogeneous predissociation. Such slight diffuseness (or broadening in absorption) is difficult to observe in our case since our line widths are limited by the laser bandwidth. The D( Y=O) level was also reported to undergo homogeneous predissociation by Herzberg and Johns [ 11. Due to the insufficient population of the higher rotational levels of the X state of CH, Herzberg and Johns did not observe the lines corresponding to P(13) of the D*II(u=2)++ X *II( v=O) transition for which, according to our observations, the line width should suddenly increase. The most plausible explanation for our data as well as those of Herzberg and Johns can be given by reference to the ab initio calculations of van Dishoeck
8 May 1992
LETTERS
[6]. Fig. 4 shows her calculated diabatic potential curves (labelled H(l), H(2) and H(3)) and diabatic vibrational levels (taken from her fig. 10 which also shows adiabatic curves and levels). We show only the diabatic curves and associated vibrational levels since the diabatic representation is more appropriate for these states. This is shown by the fact that the calculated diabatic vibrational levels agree most closely (l-20 cm-‘, see her table IX) with the position of resonances in the overall absorption spectrum calculated as simultaneous solutions of the coupled oscillator strength equations for the three states. The observation of Herzberg and Johns that the low rotational levels of the D state v=O and 2 vibrational levels both undergo homogeneous predissociation can be rationalized as induced by the repulsive valence state H ( 3 ) in fig. 4. Our observation of the apparent onset of strong rotationally dependent predissociation for N> 12 cannot be attributed to simple predissociation by the diabatic II valence state. However, the calculations of van Dishoeck suggest the foliowing explanation. The v=O level of the H(2) diabatic state is very strongly predissociated (from van Dishoeck’s fig. 12, we estimate a
hernuclear
Distance(a,)
Fig. 4. Calculated three excited *lI diabatic curves and their diabatic vibrational levels (taken from fig. 10 of ref. [ 6 1 which also shows the adiabatic curves and vibrational levels )
351
Volume 192. number 4
CHEMICAL
PHYSICS
line width of a few hundred cm-’ which would also account for the fact that this state was not observed by Herzberg and Johns nor by our MPI experiment). The II( 2)u=O level is calculated to be about 1000 cm- ’ below the D state v=2 level excluding rotation. The rotational constant for the D state v=2 level is found to be 12.6 cm-’ while we may estimate that of the II (2 ) Rydberg state to be approximately that of the ion, which is 14.2 cm-‘. The rotational levels of these two states would then become degenerate for Nx 25 and strong accidental predissociation could be expected as N approaches that value. Our data only indicates that the degeneracy occurs for N> 12 and could be at much greater values. The electronic configurations of the two states differ by two orbitals as do the avoided crossing states calculated by van Dishoeck. For these states, we can estimate from her figure that the electronic interaction matrix elements have values of about 200 cm-‘. A similar value for the II( 1) and II( 2) interaction would be sufficient to explain the experimental observation since the Franck-Condon factor should also be fairly large. However, the N= 0 energy difference between the two states should probably be decreased somewhat, e.g, by about 400-500 cm-’ which is well within the accuracy of the calculations.
4. Conclusion The influence of a nearly resonant intermediate state at the one-photon level on the two-photon rotational line strength has been observed for the (2 + 1) REMPI studies of the CH radical. The dis-
352
LETTERS
8 May 1992
crepancy between high and low laser intensity spectra has been explained by saturation effects. The apparent onset of predissociation of the D state v=2 level has been attributed to an accidental rotational predissociation through complex interactions among three II states.
Acknowledgement The authors wish to acknowledge helpful discussions with Dr. van Dishoeck. This work was supported by the National Science Foundation (CHE8821032)
References [ 11 G. Herzberg and J.W.C. Johns, Astrophys. J. I58 ( 1969) 399. [ 21 P. Chen, W.A. Chupka and SD. Colson, Chem. Phys. Letters 121 (1985) 405. [ 3 ] J.B. Pallix, P. Chen, W.A. Chupka and S.D. Colson, J. Chem. Phys. 84 ( 1986) 5208. ] P. Chen, J.B. Pallix, W.A. Chupka and S.D. Colson, J. Chem. Phys. 86 (1987) 516. ] P.J.H. Tjossem and K.C. Smyth, Chem. Phys. Letters 144 (1988) 51. 81E.F. van Dishoeck, J. Chem. Phys. 86 (1987) 196. ] Y. Wang, L. Li and W.A. Chupka, Chem. Phys. Letters 185 (1991) 478. [8] Z. Wang and H. Xia, Molecular and laser spectroscopy (Springer, Berlin, 1990). [9] C. Ma’inos, M.C. Castex and H. Nkwawo, J. Chem. Phys. 93 (1990) 5370. [lo] J.W. Hudgens, C.S. Dulcey, G.R. Long and D.J. Bogan, J. Chem. Phys. 87 ( 1987) 4546.
4
CHEMICAL
PHYSICS
LETTERS
8 May 1992
(2 + 1) resonance-enhanced multiphoton ionization studies of the CH D 213(v= 2 ) state Yumin
Wang,
Leping
Li ’ and William
A. Chupka
Sterling Chemistry Laboratory, Yale University, 225 Prospect Street, New Haven, CT 065 II, USA Received
2 I January
1992
(2 + 1) resonance-enhanced multiphoton ionization studies are performed on the D *H( v= 2) state of CH. Anomalous line intensities are shown to be due to an accidental near resonance with the C %+ state at the one-photon level. The sudden onset of predissociation for Na I2 of this level is investigated and is attributed to complex interactions among three *IT states in this energy
region.
1. Introduction In a vacuum UV study of CH, Herzberg and Johns [ I ] observed transitions to an nd Rydberg series. In addition, transitions terminating in three other states, designated D ‘I-Ii, E *II, and F 2Z+ respectively, were also observed. The D state was assigned a configuration of 2pn3 based on the sign and magnitude of its spin-orbit coupling constant. The F state was assigned to the 3po Rydberg configuration. However, the E state was left unassigned although several possible candidates were discussed. Later in a (2 + 1) REMPI study of CH, Chen et al. [ 2-41 re-examined this spectral region. Aided by the photoelectron spectrum they assigned the E state as the v=2 vibrational level of the D state and suggested that it was predissociated in some undetermined manner. The F state was not observed but instead several spectral features appeared in a longer wavelength region than the F state and were assigned to the 3po Rydberg configuration based on the appearance of the photoelectron spectrum. The latter discrepancy remains to be resolved and will not be discussed further in this publication. Recently Tjossem and Smyth [ 51 observed the MPI spectrum of the D state v=2 level under very r Present address: IBM Co., East Fishkill, Junction, New York, NY 12533, USA.
348
Rte. 52, Hopewell
0009-2614/92/$
different experimental conditions and found dramatically different relative line intensities. They suggested that these differences were caused by saturation due to a near accidental resonance at the onephoton level. Recent theoretical calculations [6] predict that three *lI states lie in the energy region of the D *H (v= 2) state. Thus complex interactions among these II states are expected and would influence the appearance of the MPI spectrum. In addition, the C state, which is a low lying valence state, falls in the one-photon energy region and therefore the two-photon transition strength to the D (u= 2 ) state will be further influenced by the C state. The purpose of this work is to verify the suggestion of Tjossem and Smyth [ 51 that the near resonance of the C state at the one-photon level influences the total (2 + 1) MPI intensity and to explain the predissociation of the D state Y= 2 level using recent calculations [ 61 of excited states of CH.
2. Experimental The experimental procedure has been described elsewhere [ 7]_ CH radicals were formed by photodissociation of bromoform with 266 nm light. CH thus produced was then probed by a frequency doubled, Nd: YAG pumped dye laser. The dye laser was operated with DCM for the spectral region of inter05.00 0 1992 Elsevier Science Publishers
B.V. All rights reserved.
Volume 192, number 4
8 May 1992
CHEMICAL PHYSICS LETTERS
est. The ion signal produced by resonance enhanced ionization of the probe laser was then amplified and collected by a Tektronix 79 12D programmable digitizer interfaced with an LSI 1 l/23 computer for mass analysis. Wavelength scans were carried out while monitoring the m/e= 13 (CH+ ) ion mass channel. All time sequences were connected by a Stanford Research System DG535 delay/pulse generator interfaced to an LSI 11/23 computer.
Q(s)
(4
I
Q(9)
Q(l'J)
Q(‘)
3. Results and discussion The D *II( u= 2) state has been observed by Chen et al. [ 21 in a (2 + 1) REMPI study of CH photodissociated from ketene in the same laser pulse. Their spectra show smooth rotational envelopes and complete 0, P, R, and S branches. In contrast to these observations, a (2 + 1) REMPI study of this state by Tjossem and Smyth [ 5 ] in a flame environment using weak laser intensity shows very different rotational line intensities. In their spectrum, only two pair of lines corresponding to the Q( 8) and Q( 9) transitions in the Q branch and several weak lines in the R branch were observed. More lines appeared when the laser intensity was increased, but the overall signal to noise ratio degraded due to increased background ionization. Noticing that the C 2C+ (v= 0) level lies close to the one-photon energy, they concluded that the lines they observed were enhanced by an accidental double resonance with the C state as the intermediate state. However, it is not clear from a comparison of the data whether the disagreement between the two sets of experiments is due to the power broadening of the C state or some other factors. Therefore, we conducted a series of experiments on the Q and R branches of the D( u= 2) t +X (v= 0) two-photon transition using different laser powers. Figs. la-lc show the spectra obtained using different laser intensities. The laser intensity was increased gradually from la to lc. It is immediately apparent that the relative intensities of the rotational lines change with laser intensity. Comparison was also made with the spectra of Tjossem et al. The two components Q1 and Q2 of the Q( 8) line which were well resolved in their spectra, were only partially resolved in fig. la which implies that our laser inten-
(cl
, ,, 4 bL__-__ 64000
64100
Two-photon
Energy
63900
(UT-~)
Fig. 1. Rotational intensity distributions of the Q branch of the DZI-I(u=2) + +X *II ( ZJ=0) two-photon transition as a function of the laser intensity. The laser intensity is increased gradually from (a) to (c).
sity was greater than theirs. Upon going from la to 1b to lc, the Q, (8) and Q2 ( 8 ) lines change from partially resolved to barely resolved to completely unresolved. This is a very strong sign of saturation either of the transition or of the signal detection or both. Thus upon increasing laser intensity, the Q ( 8 ) lines were saturated and reduced in intensity relative to other lines. Finally at very high laser intensity the spectral appearance approaches that of Chen et al. [ 21. This explanation is therefore in accordance with the tentative suggestion given by Tjossem and Smyth [ 5 ] for the differences between their spectrum and that of Chen et al. [2]. We also concur with the qualitative explanation given by Tjossem and Smyth for the extraordinarily enhanced intensity of the Q(8) line in their spectrum as being due to a near accidental coincidence 349
Volume 192, number
CHEMICAL
4
PHYSICS
(A E 7 cm-’ ) of the first photon with the R ( 8 ) line of the CtX transition followed by P( 9) of the D-C band. Fig. 2 illustrates a construction which is very convenient for a qualitative consideration of such near resonance cases [ 81. In the figure are plotted the energies of the R,, Q, and Pi branches of the C *x+ (v= 0) +-X *II( v= 0) transitions as well as the Rz, Q2 and P2 branches of the D *II( v=2)c C *C+ (v=O) transition. A schematic diagram showing those relevant transitions is presented in the lower left corner. The intervals between the upper and lower curves, e.g. between R, and P, for every N are equal to twice the values of A defined as the detunings between the intermediate state rotational energy and the one-photon energy. The relative intensities of the overall two-photon transitions thus can be inferred qualitatively from this diagram. For example, RI +P2 corresponds to the Q branch of the two-photon DttX transition, while the Q branch can also be reached via the Q1 +Q2 path. The contribution of the two paths to the Q branch intensity can be seen in the figure. The R, and P2 curves cross between N= 9 and 10 and the values of A are 7 and 34 cm-’ respectively. From the insert at the lower left comer, one can see that the path R, (N- 1) + P2 (N) corresponds to the Q (N- 1) line in the two-photon tran-
8 May 1992
LETTERS
sition. Therefore, the Q (8) and Q( 9) lines are enhanced, with Q (8) having more enhancement since it has a smaller value of A. Thus this explains the intensity of the Q branch under saturation-free condition. Experiments were also performed on the P branch ofthe two-photon D *II( ~=2)ttX *II(v=O) transition and the results are shown in fig. 3 where the spectrum of fig. 3b was taken at higher laser power than that of fig. 3a. Notice that those lines marked with an asterisk were assigned to the R branch of the E’*C+(V=O)++X*Z+(V=O) two-photon transition. The line marked with an arrow is due to the mass- 12 carbon ion contamination due to Coulomb broadening of the mass spectrum. Examination of fig. 2 shows that the Q, +P2 path is the dominant one for the P branch of the two-photon transition and the two curves cross between N= 13 and 14. Therefore, the P( 13 ) and P ( 14) lines should be enhanced by the intermediate C state. However, even at relatively low laser power, the P( 13) line intensity accounts for approximately only 20% of that of the P ( 12) line which is much less enhanced by the intermediate state. Therefore, there must be other decay channels which compete with the ionization.
(a)
I
IJ c
PilO)
2
_c
s
P(B)
p(g)
x
x
I
I
63900 5
10 Rotational
15 Quantum
20 Number
25
I
I
63700
63600
30
(N)
Fig. 2. Schematic diagram to predict the enhancement of the D211(v=2) t +X lII( v=O) two-photon transition by the nearly resonant intermediate C *Z+ (u= 0) state at the one-photon level. The insert at the lower left comer shows the relevant branches for each one photon transition. The curves represent the relevant transition energies of the branches as a function of the rotational quantum number N of the intermediate C state.
350
I
63800 Two-photon
Energy ( cm-l )
Fig. 3. Rotational intensity distributions of the P branch of the D211(v=2) ++X *II(v=O) two-photon transition as a function of the laser intensity. The spectrum shown in (b) was taken at higher laser intensity than that shown in (a). The lines marked with an asterisk were due to other transitions. The line marked with an arrow is due to the mass- 12 carbon ion contamination resulting from the Coulomb broadening.
Volume 192, number
4
CHEMICAL
PHYSICS
Furthermore the decay rate must vary greatly for different rotational levels of the D ( u= 2) state. The theoretical formalism for two-photon transition line strengths with one nearly resonant state at the one-photon level has been given by Mainos et al. [ 91. The calculation for the two-photon transition moment involves explicit considerations of complicated angular momentum couplings and is rather involved. However, the dominant factor in the calculation is the energy detuning between the onephoton energy and the energy of the intermediate state. Therefore a rough estimation can be made for the two-photon transition. For the P branch of the D(v=2) t +-X (v= 0) two-photon transition, as discussed above, the P( 13) and P( 14) should be enhanced. The ionization intensity is a function of the rotational temperature of the ground state as well as of the rotational line strength. It also depends on competing decays such as predissociation. The rotational temperature of CH formed from the photolysis of bromoform has been estimated to be roughly 2500 K [ lo]. The line strength for P( 13) can be estimated, by using only the energy detuning of the one-photon energy and the Boltzmann factor at this temperature, to be roughly five times stronger than that of the P( 12) if population decays of the upper levels are not considered. The sudden decrease in intensity of the P (N ) with N> 13 cannot be explained by a typical heterogeneous predissociation. In their VUV studies, Herzberg and Johns [ 1 ] observed that the D( v= 2 ) +X (EtX in their notation) band shows a slight diffuseness which did not vary with J from which they concluded that the E ‘II is subject to a homogeneous predissociation. Such slight diffuseness (or broadening in absorption) is difficult to observe in our case since our line widths are limited by the laser bandwidth. The D( Y=O) level was also reported to undergo homogeneous predissociation by Herzberg and Johns [ 11. Due to the insufficient population of the higher rotational levels of the X state of CH, Herzberg and Johns did not observe the lines corresponding to P(13) of the D*II(u=2)++ X *II( v=O) transition for which, according to our observations, the line width should suddenly increase. The most plausible explanation for our data as well as those of Herzberg and Johns can be given by reference to the ab initio calculations of van Dishoeck
8 May 1992
LETTERS
[6]. Fig. 4 shows her calculated diabatic potential curves (labelled H(l), H(2) and H(3)) and diabatic vibrational levels (taken from her fig. 10 which also shows adiabatic curves and levels). We show only the diabatic curves and associated vibrational levels since the diabatic representation is more appropriate for these states. This is shown by the fact that the calculated diabatic vibrational levels agree most closely (l-20 cm-‘, see her table IX) with the position of resonances in the overall absorption spectrum calculated as simultaneous solutions of the coupled oscillator strength equations for the three states. The observation of Herzberg and Johns that the low rotational levels of the D state v=O and 2 vibrational levels both undergo homogeneous predissociation can be rationalized as induced by the repulsive valence state H ( 3 ) in fig. 4. Our observation of the apparent onset of strong rotationally dependent predissociation for N> 12 cannot be attributed to simple predissociation by the diabatic II valence state. However, the calculations of van Dishoeck suggest the foliowing explanation. The v=O level of the H(2) diabatic state is very strongly predissociated (from van Dishoeck’s fig. 12, we estimate a
hernuclear
Distance(a,)
Fig. 4. Calculated three excited *lI diabatic curves and their diabatic vibrational levels (taken from fig. 10 of ref. [ 6 1 which also shows the adiabatic curves and vibrational levels )
351
Volume 192. number 4
CHEMICAL
PHYSICS
line width of a few hundred cm-’ which would also account for the fact that this state was not observed by Herzberg and Johns nor by our MPI experiment). The II( 2)u=O level is calculated to be about 1000 cm- ’ below the D state v=2 level excluding rotation. The rotational constant for the D state v=2 level is found to be 12.6 cm-’ while we may estimate that of the II (2 ) Rydberg state to be approximately that of the ion, which is 14.2 cm-‘. The rotational levels of these two states would then become degenerate for Nx 25 and strong accidental predissociation could be expected as N approaches that value. Our data only indicates that the degeneracy occurs for N> 12 and could be at much greater values. The electronic configurations of the two states differ by two orbitals as do the avoided crossing states calculated by van Dishoeck. For these states, we can estimate from her figure that the electronic interaction matrix elements have values of about 200 cm-‘. A similar value for the II( 1) and II( 2) interaction would be sufficient to explain the experimental observation since the Franck-Condon factor should also be fairly large. However, the N= 0 energy difference between the two states should probably be decreased somewhat, e.g, by about 400-500 cm-’ which is well within the accuracy of the calculations.
4. Conclusion The influence of a nearly resonant intermediate state at the one-photon level on the two-photon rotational line strength has been observed for the (2 + 1) REMPI studies of the CH radical. The dis-
352
LETTERS
8 May 1992
crepancy between high and low laser intensity spectra has been explained by saturation effects. The apparent onset of predissociation of the D state v=2 level has been attributed to an accidental rotational predissociation through complex interactions among three II states.
Acknowledgement The authors wish to acknowledge helpful discussions with Dr. van Dishoeck. This work was supported by the National Science Foundation (CHE8821032)
References [ 11 G. Herzberg and J.W.C. Johns, Astrophys. J. I58 ( 1969) 399. [ 21 P. Chen, W.A. Chupka and SD. Colson, Chem. Phys. Letters 121 (1985) 405. [ 3 ] J.B. Pallix, P. Chen, W.A. Chupka and S.D. Colson, J. Chem. Phys. 84 ( 1986) 5208. ] P. Chen, J.B. Pallix, W.A. Chupka and S.D. Colson, J. Chem. Phys. 86 (1987) 516. ] P.J.H. Tjossem and K.C. Smyth, Chem. Phys. Letters 144 (1988) 51. 81E.F. van Dishoeck, J. Chem. Phys. 86 (1987) 196. ] Y. Wang, L. Li and W.A. Chupka, Chem. Phys. Letters 185 (1991) 478. [8] Z. Wang and H. Xia, Molecular and laser spectroscopy (Springer, Berlin, 1990). [9] C. Ma’inos, M.C. Castex and H. Nkwawo, J. Chem. Phys. 93 (1990) 5370. [lo] J.W. Hudgens, C.S. Dulcey, G.R. Long and D.J. Bogan, J. Chem. Phys. 87 ( 1987) 4546.