1727
J. Phys. Chem. 1995, 99, 1727-1732
Ion Distributions for Resonance Enhanced Multiphoton Ionization of C10 Kwanghsi Wang* and V. McKoy Arthur Amos Noyes Laboratory of Chemical Physics,? Califonia Institute of Technology, Pasadena, Califomia 91125 Received: August 15, I994@
Results of theoretical studies of rotationally resolved photoelectron spectra for (24- 1’) resonance enhanced multiphoton ionization (REMPI) of the C %, D 22-,E 22-,and F 2Z- Rydberg states of C10 are presented. Cooper minima are predicted to occur in the electronic continua for photoionization of the D and F states and lead to unusual behavior in the ion rotational distributions. Strong partial wave I mixing is also predicted in the continua for the C and E states due to the nonspherical molecular ion potentials. These Cooper minima and 1 mixing make the associated photoionization dynamics quite nonatomiclike.
Introduction The halogen monoxide radicals C10 and BrO are of great interest due to their central role in the C10, and BrOx cycles of stratospheric ozone depletion. 1-5 Of the many spectroscopic studies of these radicals, the most relevant to the present work are those of Duignan and Hudgem6 They reported resonance enhanced multiphoton ionization (REMPI) spectra of C10 and BrO via three-photon resonant transitions to several Rydberg states. Some of these Rydberg states had been previously identified by Basco and Morse7 using vacuum ultraviolet absorption spectra. In this paper we report calculated rotationally resolved photoelectron spectra for (2+1’) REMPI via the C, D, E, and F Rydberg states of C10 leading to the X 32-ground state of the C10+. Photoelectron spectra are reported for photoelectron energies of 50 meV and 2 eV. The spectra at 50 meV provide ion rotational distributions that could be readily measured by the zero-kinetic-energy (ZEKE) technique, where photoelectrons resulting from pulsed-field ionization of very high Rydberg levels are d e t e ~ t e d . ~The , ~ spectra at 2 eV should characterize the ion rotational distributions for more conventional photoelectron energies. Some of these ion rotational distributions show quite interesting behavior due to strong mixing of angular momenta in the photoelectron orbital and to the presence of Cooper minima. We hope that these calculated spectra will stimulate measurements of these REMPI ion rotational distributions. Such studies could prove valuable in the use of REMPI for state-selective detection and studies of these important species.
Theory and Numerical Details (2+1’) REMPI processes via the C, D, E, and F 22intermediate Rydberg states of C10 are viewed as two-step processes with a two-photon absorption from an initially unaligned X 211ground state (all Mj0 levels equally populated) to these resonant intermediate states, followed by subsequent one-photon absorption from the aligned resonant states to the ionization continuum. Under collision-free conditions, each MJ channel can be treated as an independent ionization channel for linearly polarized light. The differential cross section for photoionization of the resonant states by the third photon can be written as Contribution No. 897 1. @Abstractpublished in Advance ACS Abstracts, January 15, 1995. +
0022-365419512099- 1727$09.OO/O
where u is the total cross section, PL(COS0) the Lth Legendre polynomial, and / 3 the ~ asymmetry parameters. The formulation for obtaining the total cross section 0 and asymmetry parameters PL has been given previously for Hund’s case (b) coupling scheme.l03l1 A central quantity in our studies is the photoelectron matrix element for photoejection of an electron from a bound molecular orbital & into a photoelectron continuum orbital +&)(r). Here k is the momentum of the photoelectron and (-) denotes incoming-wave boundary conditions. Single-center expansions of 4i and Y&)(r) define the photoelectron matrix element as
where R denotes a dependence on internuclear distance, p the photon polarization index in the molecular frame, and 2 the projection of 1 in the molecular frame. Whereas only 1 = 1’ would be allowed in eq 2 for the central fields of atomic systems, where the angular momentum of the photoelectron must be conserved, 1 t Z’ terms arise in the molecular case due to angular momentum coupling brought about by the nonspherical fields of these ions. These angular momentum changing collisions between the photoelectron and molecular ion play a crucial role in rotationally resolved photoelectron spectra. The use of molecular photoelectron orbitals which correctly incorporate the angular momentum coupling is essential in such studies. There are two dipole-allowed transition channels for photoionization of the 80, 90, 100, and l l u orbitals of the C 2Z-, D 22-,E 22-,and F 22-Rydberg states, respectively. An electron can be ejected from these Rydberg orbitals into the ko or kn continuum channel. Ejection through the ka continuum results in an electron-ion complex which has 22- total final-state symmetry while ejection through the kn continuum results in an electron-ion complex of symmetry. The final-state wave functions are
-
-
I(core)3n+3n-kal - I(core)3n+3n-koll (3) 0 1995 American Chemical Society
1728 J. Phys. Chem., Vol. 99, No. 6,I995
Wang and McKoy
-
0.5
Y(~II> = (1/~)[2l(core)3n+3x_kjt,l-
-
I
-
I( c o r e ) 3 n + 3 n ~ nI+- I(core)Zk+3n._kn+I1
(4)
(2Ji - Ki) i=core
+ a,,J,, -I- b,K,
aMM3 P = 4
?
2
--a
0.3
W
14 0.2 0.1
- E ) P ~ &= ) 0 (5)
o where Ji and Ki are the Coulomb and exchange operators, respectively, and P is a projection operator which enforces orthogonality of the continuum orbital to the occupied orbitals.12 The photoelectron kinetic energy is given by E = V2k2 and the one-electron operator f is
f = -Y2V? - Cza/ria
.
o
r
2.0
~
~
8.0
4.0
!
l
8.0
~
l
10.0 I
I
-P=l
Mnm P=2
0.4 h
5(d
0.3
-
v
(6)
n
14 0.2
with Za a nuclear charge. Using the wave functions of eqs 3 and 4, the coefficients a,, and b, associated with the 3n orbital assume values of 1 and l/2, respectively. To obtain the photoelectron orbitals, we have used an iterative procedure, based on the Schwinger variational principle,12 to solve the Lippmann-Schwinger equation associated with eq 5. In this procedure, the static-exchange potential of the ion is approximated by
C(riulai)(rl)ij(ajl ulr’)
0.0
0.5
a
u(r,r’>=
u=2 U P=3 vyyyy
0.4
where (core) = 1022023a24025a26a27a21d2d?t’. Within the frozen-core Hartree-Fock approximation,the oneelectron Schrijdinger equation for the photoelectron orbital #k associated with the wave functions of eqs 3 and 4 can be shown to have the form12
P(f+
I
t+Lv P=o :O P =:1l
(7)
9
0.1 0.0 0.0
2.0
6.0
4.0
8.0
10.0
Kinetic Energy (eV) Figure 1. Magnitude lDf-)l of the partial wave components of the photoelectron matrix element for photoionization of the C zX- (80) Rydberg state of C10 for the (a) 80 ka and (b) 80 kn ionization
channels.
-
-
ij
where the matrix U-’ is the inverse of the matrix with element (v)= ~ (ai(Ulaj)and the a’s are discrete basis functions such as Cartesian or spherical Gaussian functions. U is twice the static-exchange potential in eq 5 with the long-range Coulomb potential removed. The Lippmann-Schwinger equation with this separable potential U(r,r’) can be readily solved and provides approximate photoelectron orbitals 4;)’. These solutions can be iteratively improved to yield converged solutions to the Lippmann-Schwinger equation containing the exact static-exchange potential. In this study, two iterations provided converged wave functions for eq 5. All matrix elements used in the solution of Lippmann-Schwinger equation were evaluated via single-center expansions about the center of mass. The radial integration grid extends to 64 au and contained 800 points. The integration step sizes ranged from 0.01 to 0.16 up to 16 au, and up to 0.16 au beyond this point. We used the improved virtual orbital method13 to obtain the wave functions of the C 2C- ( 3 ~ 8 o )D, 22-( 3 ~ 9 4E, 22( 3 ~ 1 0 0 ) and , F 22-( 3 ~ lo) 1 resonant Rydberg states. The core orbitals are taken to be those of the fully relaxed 32-ion. The orbital basis used in these calculations consists of a [6s,5p] contraction of the (12s,9p) primitive Cartesian Gaussian basis of Dunning,14 augmented by one s (a= 0.1) and two d (a= 0.85 and 0.1) functions on the chlorine atom. On the oxygen atom we used the [5s,3p/3s,2p] basis of contracted Cartesian Gaussian functions15augmented with two d (a= 0.85 and 0.1) functions. This basis was further augmented with four s (a= 0.05, 0.02, 0.008, and 0.004), four p (a = 0.05, 0.02, 0.008, and 0.0041, and three d (a= 0.055,0.015, and 0.008) functions at the center of mass. With this basis set, we obtained total energies of -534.028 436, -534.003 087, -533.956 034, and -533.940025 au for the C, D, E, and F Rydberg states,
respectively, at an intemuclear distance of R = 2.797 74 au which is the Re of the X 32-ground state of C10+.
Results and Discussion
(2+1’) REMPI via the C 2E-Rydberg State. The C 22( 3 ~ 8is~the) first Rydberg state of the C10 and is generally thought to be a 3da Rydberg Single-center expansion of the 8a orbital about the center of mass shows that it has strong p and d character [14.56% s, 39.75% p, 36.58% d, 6.10% f, 1.00% g (lo = 4), and 0.74% h (lo = 5)] at an internuclear distance of 2.797 74 au. Due to the core penetration of this o orbital, its angular momentum composition can be expected to change with internuclear distance. Indeed, the 8o orbital has dominant 4s character at small R and a strong d component with comparable contributions from s, p, and f waves at larger R. For example, the 80 orbital has the following composition: 85.10% S, 1.20% p, 13.10% d, 0.17% f, 0.25% g (lo = 4), and 0.03% h (lo = 5) at R = 2.25 au, 45.42% s, 21.34% p, 29.42% d, 2.44% f, 0.51% g (lo = 4), and 0.26% h (lo = 5) at R = 2.65 au, and 18.64% s, 22.85% p. 38.72% d, 10.70% f, 3.49% g (10 = 4), and 2.10% h (lo = 5) at R = 3.5 au. To understand the underlying behavior of the rotationally resolved photoelectron spectra, it is useful to examine the angular momentum composition of the photoelectron matrix elements. Figure 1 shows the magnitude of the (incoming-wave normalized) partial wave dipole amplitude lDj-)l as a function of photoelectron kinetic energy for the (a) 80 ko and (b) 8o kn photoionization channels of the C 2Z-Rydberg state of C10 at R = 2.797 74 au. The d (I = 2) partial wave component is seen to be dominant near threshold while the s ( I = 0), p (1 = l), and f (1 = 3) waves are much less intense. With the dominant p (39.75%) and d (36.58%) character of the So orbital,
-
-
~
l
r
Ion Distributions for REMPI of C10
J. Phys. Chem., Vol. 99, No. 6, 1995 1729 on
1.0
0
,
I
0.8
.rl
.e
ld
Kc
2
3
h
j 0.2 cd v
0.6
-
h
L4
0.4
a
ld
m
0.1
0.2 0.0
10.0
5.0
15.0
20.0
0.0
25.0
,
2.0
0.0
2.0
0.0 0.2
4.0
6.0
8.0
10.0
1
0 0.8
3
e: 0.6
-3
0.1
u
5.0
l0:O
15.0
2010
25:O
Rotational Energy (mev) Figure 2. Calculated rotationally resolved photoelectron spectra and associated photoelectron angular distributions for (2+ 1') REMPI via the Sll(10) branch of the C zZ- Rydberg state of C10: (a) for an energy of 50 meV; (b) for an energy of 2 eV. See text for explanation. these s, p, and f contributions to the photoelectron matrix element would not be expected to be so much weaker than that of the d wave on an atomiclike basis. The small magnitude of the f wave component near threshold suggests that a Cooper minimum, not seen above threshold, may occur in the discrete region. Such a Cooper minimum has been identified in the optical-optical double resonance spectra via the C 211Rydberg state of N0.16 At higher energy, there is strong 1 mixing, especially between the d and f waves. It is also important to note that the g ( I = 4) wave increases steadily as the kinetic energy increases and becomes one of the most important angular momentum components of the photoelectron matrix element at 10 eV. This behavior of the g (1 = 4) is entirely nonatomiclike. In Figure 2a we show the rotationally resolved threshold ZEKE photoelectron spectrum and associated photoelectron angular distributions for (2+1') REMPI via the Sll(10) rotational branch of the C 21:- Rydberg state of C10. In these calculations, a photoelectron energy of 50 meV is assumed and the spectrum is convoluted with a Gaussian detector function having a full width at half-maximum (fwhm)of 1 meV. The labels AN (=PI+ - W ) designate the angular momentum change, exclusive of spin, upon ionization. For this S11(10) branch, the AN = 0 peak hence corresponds to the N+ = 12 rotational level of the ion. The most intense transition of the photoelectron spectrum is normalized to unity. Asymmetry parameters up to ,& are included in the calculation of the photoelectron angular distributions. The photoelectron angular distributions with AN < 0 are not shown since they are similar to their positive AN counterparts. For the photoelectron angular distributions, we have assumed PO= 1. In Figure 2b we show the ion rotational distributions and associated photoelectron angular distributions for (2fl') REMPI via the Sll(10) branch of the C 2C- state of C10 at a
4.0
6.0
8.0
10.0
Kinetic Energy (eV)
Figure 3. Magnitude 1Di-'1 of the partial wave components of the photoelectron matrix element for photoionization of the D (94 Rydberg state of C10 for the (a) 90 ko and (b) 9o h ionization channels. The insets show the principal-value dipole amplitude Dr for the I = 1 and 2 components. +
+
photoelectron kinetic energy of 2 eV. These spectra as well as the ZEKE spectra of Figure 2a show dominant AN = odd transitions with AN = 1 being the most intense transition. This can be readily understood on the basis of the parity selection rule of AN 1 = 0dd,'~9''9'~which is applicable for this Z - I: transitions. The partial wave composition of photoelectron matrix element of Figure 1 indicates that the d ( I = 2) component is dominant at both 50 meV and 2 eV. On the basis of the parity selection rule, this leads to the dominant AN = odd transitions. On the other hand, the AN = even transitions are considerably weaker than the AN = odd transitions both at 50 meV and 2 eV. The increase in the intensity of the AN = even contributions at 2 eV arises from the increase in the f (1 = 3) contribution to the photoelectron matrix element at higher energy (about 4 eV) accompanied by a decrease in the contribution of the d (1 = 2) wave (see Figure 1). On the basis of parity selection rules and the partial wave composition of the 8a orbital (almost equal p and d contributions), atomiclike propensity rules would suggest that the intensities of the odd and even peaks would be quite similar. For similar reasons, dominant even transitions would be expected if the C 21:- Rydberg state had 3da character. The ion rotational distributions of Figure 2 clearly behave differently. The spectral profiles of Figure 2 indicate that interference among the partial waves is important. This is particularly evident for the AN = 3 peaks of Figure 2. (2+1') REMPI via the D *I:- Rydberg State. The D 2ZRydberg state of C10 has the electron configuration (ion core)9a. The character of this 9a orbital also changes significantly with internulcear distance. Single-center expansion of the 90 orbital reveals that it has dominant 4p character at smaller internuclear distance and strong 4s character at larger R . The angular
+
1730 J. Phys. Chem., Vol. 99, No. 6, 1995
Wang and McKoy 0.3
I ttLv P=O -P=l
OnUuD P=2
4A&a
P=3
QMXX) P=4
j 0.2 rd W
--
iq
a
0.1
0.0
2.0
0.0
2.0
4.0
6.0
8.0
10.0
4.0
6.0
8.0
1’ I
3
1
Kinetic Energy (eV)
Figure 4. Calculated rotationally resolved photoelectron spectra and associated photoelectron angular distributions for (2+ 1’) REMPI via the S11(10)branch of the D ?Z- Rydberg state of C10: (a) for an energy of 50 meV; (b) for an energy of 2 eV. See text for explanation.
Figure 5. Magnitude /Di-)l of the partial wave components of the photoelectron matrix element for photoionization of the E 22-(100) Rydberg state of C10 for the (a) 100 ku and (b) 100 lut ionization channels.
momentum composition of this orbital at several internuclear distances is 7.42% s, 72.93% p, 18.40% d, 0.82% f, 0.11% g (lo = 4),and 0.09% h (lo = 5 ) at R = 2.25au, 48.34% s, 43.58% p, 5.38% d, 1.84% f, 0.31% g (lo = 4), and 0.24% h (lo = 5 ) at R = 2.65 au, and 95.17% s, 0.50% p, 3.30% d, 0.28% f, 0.45% g (lo = 4), and 0.11% h (lo = 5 ) at R = 3.5 au. At the internuclear distance of 2.797 74 au of interest here, the 9a orbital has 81.31% s, 15.73% p, 1.12% d, 1.10% f, 0.36% g (lo = 4), and 0.18% h (lo = 5 ) character. Figure 3 shows the magnitude of the (incoming-wave normalized) partial wave dipole amplitude IDj-’I as a function of photoelectron kinetic energy for the (a) 90 ka and (b) 9a k z photoionization channels of the D 22-Rydberg state of C10 at R = 2.79774 au. The d (1 = 2)partial wave component is dominant in the ka channel while the p wave is dominant in the h channel (except for the near-threshold region). The dominance of the d ( I = 2) wave in the ka and h (nearthreshold) continua is very anomalous since a strong p (1 = 1) partial wave contribution to the photoelectron matrix element would be expected on the basis of atomiclike propensity rules and the almost pure s (81.31%) character of the 90 orbital. Examination of the photoelectron matrix element reveals that two Cooper minima, which are due to sign changes in the p (1 = 1) wave of the ka continuum and in the d (I = 2) component of the k n channel, are formed upon ionization. Depletion of the p wave in the k a continuum leads to a relative enhancement of the d wave contribution to the matrix element whereas depletion of the d wave in the h continuum strongly reduces its contribution to the matrix element at higher photoelectron energies. Closer to threshold, strong 1 coupling enhances the d-wave component of the kn continuum. The actual sign changes in the matrix elements occur in the principal-value
(standing-wave normalized) dipole amplitude Of, shown in the insets of Figure 3. In Figure 4 we show the rotationally resolved photoelectron spectra and associated photoelectron angular distributions for (2+1’) REMPI via the SlI(10) rotational branch of the D 22Rydberg state of C10 at photoelectron energies of 50 meV (Figure 4a) and 2 eV (Figure 4b). At both energies, dominant AN= odd transitions are clearly seen in contrast to the expected AN = even transitions on the basis of an atomiclike propensity rule. The low intensity of the AN = even transition is due to the Cooper minimum in the p wave of the ka continuum. The depletion of the p wave around the minimum of the ID;-’I substantially enhances the contribution of the d wave. Similar effects of Cooper minima on rotationally resolved photoelectron spectra have also been observed for REMPI of the D 2C- of NO,’* the D 2Z- state of OH,19 and the f ill state of NH.20 Such Cooper minuma associated with photoionization of Rydberg states of molecules were first predicted in (3fl)REMPI of the D 2C- state of OH by Stephens and M ~ K o y . ~ l (2+1’) REMPI via the E 2Z- Rydberg State. The E % Rydberg state of C10 has the electron configuration (ion core)100. This 100 orbital also evolves rapidly from an s, p, and d admixture at smaller internuclear distance to dominant 4p character at larger R. The angular momentum composition of the 100 orbital at several internuclear distances is as follows: 38.54% S, 33.59% p, 26.95% d, 0.80% f, 0.02% g (lo = 4),and 0.05% h (10 = 5 ) at R = 2.25 au, 21.69% s, 57.93% p, 17.81% d, 2.08% f, 0.12% g (lo = 4),and 0.20% h (lo = 5 ) at R = 2.65 au, and 0.54% s, 96.37% p, 1.30% d, 1.16% f, 0.20% g (IO = 4), and 0.22% h (10 = 5) at R = 3.5 au. At the internuclear distance of 2.79774 au of interest here, the 10a orbital has 11.36% s, 73.63% p, 11.56% d, 2.67% f, 0.22% g (lo = 4),and 0.30% h (IO = 5 ) character.
-
-
-
-
Ion Distributions for REMPI of C10
0
J. Phys. Chem., Vol. 99, No. 6,I995 1731
0.8
+
.d
(d
p:
2
0.6
-
1
0.4
2 m
I - 4
a
0.1 0.1
& \
0.2 0.0 0 .0; 0.0
0.0
5.0
10.0
1.0
15.0
20.0
25.0
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,
I
4.0
,
,
1
I
8.0
8.0
I
10.0
0
AN=l 0 0.8
I
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0.2
+
1
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-
L-4
a
0.01
5.0
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20:O
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Figure 5 shows the magnitude of the (incoming-wave normalized) partial wave dipole amplitude IDj-’I as a function of photoelectron kinetic energy for the (a) 10a ka and (b) 100 kn photoionization channels at R = 2.797 74 au. The dominance of the d ( I = 2) partial wave component in both the ka and kn channels is consistent with an atomiclike picture of the photoionization of this orbital (73.63% p character). However, the unusually strong p wave near-threshold in the ka continuum and the magnitude of the f and g (1 = 4) waves at larger photoelectron energies show that the overall photoionization dynamics are quite nonatomiclike. This behavior results from strong 1 mixing in the continuum caused by the nonspherical molecular ion potential. Minima are seen in the f waves near-threshold in both ionization channels; however, no Cooper zeros were found in the energy range studied. In Figure 6 we show the rotationally resolved photoelectron spectra and associated photoelectron angular distributions for (2+1’) REMPI via the S11(10) rotational branch of the E *ERydberg state of C10 at photoelectron energies of 50 meV (Figure 6a) and 2 eV (Figure 6b). At 50 meV the AN = 0 peak is the strongest and the intensities of the AN = even and AN = odd transitions are comparable. This is not expected on an atomiclike basis for photoionization of an orbital with dominant p character. On the other hand, dominant AN = odd transitions are predicted at a photoelectron energy of 2 eV in line with an atomiclike picture even though the contributions of the odd waves (AN = even transitions) are slightly larger. The photoelectron angular distributions also display the dependence of the angular momentum composition of the photoelectron wave function on kinetic energy. (2+1’) REMPI via the F 2X- Rydberg State. The F *ERydberg state of C10 has the electron configuration (ion core)1lo. This 1la orbital also evolves from a dominant s and p
-
-+
I
,
!
4.0
,
8.0
1
I
8.0
I
I
10.0
Kinetic Energy (eV)
Rotational Energy (mev) Figure 6. Calculated rotationally resolved photoelectron spectra and associated photoelectron angular distributions for (2+ 1’) REMPI via the S11(10) branch of the E 2X- Rydberg state of C10: (a) for an energy of 50 meV; (b) for an energy of 2 eV. See text for explanation.
,
2.0
Figure 7. Magnitude lDj-)l of the partial wave components of the photoelectron matrix element for photoionization of the F 2X- (1la) Rydberg state of C10 for the (a) 1la k a and (b) 1la krc ionization channels. The insets show principal-value dipole amplitude 0: for the 1 = 1 and 3 components.
-
-
admixture with stronger s character at smaller internuclear distance to a dominant s and d admixture with stronger d character at larger R. For example, the angular momentum composition of the 1lo orbital is as follows: 68.18% s, 24.56% p, 6.81% d, 0.31% f, 0.07% g (lo = 4), and 0.04% h (lo = 5) at R = 2.5 au and 37.90% s, 0.22% p, 61.72% d, 0.09% f, 0.05% g (lo = 4), and 0.01% h (lo = 5) at R = 3.5 au. At the internuclear distance of 2.797 74 au of the interest here, this orbital has 49.77% s, 16.61% p, 32.53% d, 0.78% f, 0.11% g (lo = 4), and 0.09% h (lo = 5) character. Figure 7 shows the magnitude of the (incoming-wave normalized) partial wave dipole amplitude lDj-)l as a function of photoelectron kinetic energy for the (a) lla ka and (b) 1lo kn photoionization channels of the F *E-Rydberg state of C10 at R = 2.797 74 au. Dominant d ( I = 2) partial wave components are seen in both the ka and kn channels in contrast to the p and f waves expected on the basis of atomiclike behavior for an orbital with dominant s (49.8%) and d (32.5%) character. This behavior is due to Cooper minima in the p and f waves of the ka channel and in the f wave of the kx channel. The associated principal-value dipole amplitudes are shown in the insets of Figure 7. Note that the p wave of the kn continuum has a minimum near-threshold; however, no Cooper zeros are detected in this component in the energy range studied. These Cooper minima will play an important role in rotationally resolved photoelectron spectra due to the depletion of the associated angular momentum components of the photoionization matrix element. In Figure 8 we show the rotationally resolved photoelectron spectra and associated photoelectron angular distributions for (24-1’) REMPI via the Sll(10) rotational branch of the F 22Rydberg state of C10 at photoelectron energies of 50 meV
-
+
1732 J. Phys. Chem., Vol. 99, No. 6, 1995 1.0 0 0.8
3
ld 12:
6"
2
0.6 0.4
E D 0.2 0.0
Wang and, MCKOY for (2+1') REMPI of the C, D, E, and F 2Z- Rydberg states are reported. Unusually strong AN = odd transitions are predicted for these Rydberg states. This unusual behavior arises from the presence of several Cooper minima in the ionization channels for the D and F states and strong 1 mixing in the electronic continuum due to the nonspherical potential of the molecular ions. Our studies have identified the rich dynamics of quantum-state specific studies of molecular photoionization of this halogen monoxide and should serve to stimulate some experimental investigations of this system.
Acknowledgment. This work was supported by grants from the National Science Foundation, Air Force Office of Scientific Research, and the Office of Health and Environmental Research of the U.S.Department of Energy. We also acknowledge use of the resources of the Jet Propulsion LaboratoryICalifomia Institute of Technology CRAY Y-MP2El232 Supercomputer. References and Notes (1) Rowland, F. S.; Molina, M. J. Rev. Geophys. Space Phys. 1975, 13, 1.
Figure 8. Calculated rotationally resolved photoelectron spectra and associated photoelectron angular distributions for (2+ 1') REMPI via the S11(10)branch of the F *Z-Rydberg state of CIO: (a) for an energy of 50 meV; (b) for an energy of 2 eV. See text for explanation.
(Figure 8a) and 2 eV (Figure 8b). Both spectra show dominant m = odd transitions due to the strong d wave contribution to the photoelectron matrix element at both energies. The m = even transitions at 2 eV are considerably weaker than those at 50 meV since the former energy is located within these Cooper minima. Photoelectron angular distributions also reflect such behavior especially for the AN = k2 transitions.
Conclusion Results of calculations of rotationally resolved photoelectron spectra and their associated photoelectron angular distributions
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