J . Phys. Chem. 1986, 90, 274-278
274
I I ~
!
i ~
( c ) By Benzoquinone and Naphthoquinone. The quenching of ZnPP by benzoquinone was measured in ethanol solution since benzoquinone does not dissolve readily in the H20-EtOH mixture and easily forms a cloudy solution. The decay rate constant found is k, = 7 X lo9 M-’ s-’. The activation energy for the process obtained from the temperature dependence of the quenching rate constant is E , E 3 kcal/mol as can be expected for a diffusioncontrolled reaction. The quenching rate constant for naphthoquinonesulfonate in our ethanol-water mixture was k, = 8 X lo8 M-I SKI. In conclusion it can be said that Zn protoporphyrin in monomeric dispersion in an aqueous ethanol solvent behaves very ~~
0 0
1
I
I
I
-
Vibrational Analysis of the A-X Photodissociation Spectrum of CH,I+ Anne M. Woodward,+ Steven D. Colson,* William A. Chupka, Sterling Chemistry Laboratory, Yale University, New Haven, Connecticut 0651 1
and Michael G. White Department of Chemistry, Brookhaven National Laboratory, Upton, New York 11973 (Received: August 12, 1985)
Methyl iodide ions are prepared in the 2111,2 and 2113/2spin states by multiphoton ionization. Resonant enhancement via low-lying Rydberg states is used to produce ions in a controlled, narrow distribution of vibrational levels as shown by the photoelectron spectra obtained at the selected ionization wavelengths. The photodissociation spectra of-CH31+_andCD31+ are obtained in a time-of-flight-massspectrometer and analyzed to obtain the vibrational constants for X and A states and the spin-orbit splitting of the X state.
Introduction We reported previously on the use of multiphoton ioniza_tio_n to prepare CH31+ in selected vibrational states.] The A-X photodissociation spectrum was then obtained by using pumpprobe, two-laser time-of-flight mass spectroscopy. The resulting spectrum was much simpler than that obtained2 when electron bombardment was used to prepare the ions. In this paper we report the use of photoelectron spectra to characterize the ionization process and obtain the spectra of CH31+in several different initial states to obtain improved values for the vibrational and spin-orbit splitting constants. The multiphoton ionization (MPI) spectrum of methyl iodide taken in a static cell in the region from 360 to 41 5 nm has been reported by Parker et al.3 They have identified two two-photon electronic transitions in the energy region of our interest. These correspond to excitation of a nonbonding iodine 5p electron to a 6s molecular Rydberg orbital on an ion core of the ground-state configuration. The bare ion core in this configuration has 211112 and 2113/2 state (in C,, symmetry) which are separated by -5000 cm-’ because of strong spin-orbit coupling. An energy level diagram for methyl iodide showing the four vibronic levels of interest is given in Figure 1. It is important to note that while all four levels are nominally of the 211,,2core, addition of a third photon above the two-photon resonance results in a total energy Molecular Spectroscopy Division, National Bureau of Standards, Gaithersburg, MD 20899.
0022-3654/86/2090-0274$01 S O / O
which is above the ’II3/2 ionization potential for all four levels but also above the *IIIl2ionization potential for only two of the levels (II(0,O) and II(0,l)). In fact, the II(0,O) level is, accidently, ionization limit. This exactly two-thirds of the energy of the 211112 portion of the spectrum was selected for study because it presents an opportunity for the study of the propensity for conservation of vibrational and/or spin quantum numbers during the ionization step. The four resonant levels chosen for ionization represent two electronic states, each nominally with the excited 211ij2core. For each electronic state, one level has only zero-point vibrational energy and the other level has, in addition, one quanta of u2 excitation. An accumulating body of evidence from our laboratory and others suggests that if the resonant Rydberg state is well described by the “ion core plus weakly interacting Rydberg electron model”4 and by a single electronic and vibrational configuration, single-photon ionization will strongly preserve the ion core electronic and vibrational quantum numbers. Neglecting (1) W. A. Chupka, S. D. Colson, M. S. Seaver, and A . M . Woodward, Chem. Phys. Leu.,95, 171 (1983). (2) S. P. Goss, D. C. McGilvery, J. D. Morrison, and D. L. Smith, J . Chem. Phys. 75, 1820 (1981); S. P. Goss, J. D. Morrison, and D. L . Smith, J . Chem. Phys., 75, 757 (1981). (3) D. H. Parker, R. Pandolfi, P. R. Stannard, and M. A. El-Sayed, Chem. Phys., 45, 27 (1980). (4) M. Mastsuzawa, J . Phys. SOC.Jpn., 32, 1088 (1972); J . Phys. B, 8, 2144 (1975), and references therein.
0 1986 American Chemical Society
A-R
The Journal of Physical Chemistry, Vol. 90, No. 2, 1986 275
Photodissociation Spectrum of CHJ' 96,369 cm"
x
TABLE I: Positions of the Observed Maxima in the Two-Photon Resonance-Enhanced MPI Swctrum of CHJ and CDJ X, nm assignt
CH31 370.25 369.94 365.87 362.59
CDiI 369.44 369.24 365.02 363.90 363.70 363.65
(VI3V2)
W O )
W,O) Z(0,l)
358.84
n(o,l)
TABLE 11: MPI-PE Swctra of Methvl Iodide obsd energy, A, nm intermed state eV 369.94 Z(0,O) 0.52 0.35 362.59 W1) 0.72 0.56 0.4 365.87 n(o,o) 0.63 0.48 0.32 358.84 n w ) 0.8 0.64 0.49 0.31 Figure 1. Energy level diagram for methyl iodide. Note that the figure is not drawn to scale.
i
la
i
(a1
l
CHJ* state
2 2n3/2
g
2 I I , I 2 , V2 2n3/2 2n3/23 v2
= 1
Y2
= 1 =2
ic 2 ~ 3 / 2 .~2
= 1
2 n 3 / 2 , ~2
=2
2n3/2 2n,/2, u2
=1
ic 2 I I 3 / 2 , 2n3/2
8
ic ic 2 n , / 2 ,
2 2n,/2,
Y2 Y2
=2 =3
(a 1
m
a a
d
(b)
2
C
0.0 35 7.0 0
371.00 WAVELENGTH (nm)
Figure 2. MPI spectrum of (a) methyl iodide and (b) methyl-d3iodide. Labeling of the peaks corresponds to the labeling of the transitions in the energy level diagram of Figure 1.
total energy constraints, one might therefore expect that the four chosen resonant levels would produce 21111,2 ground-state ions with either zero-point vibrational energy or one quanta of u2. However, in our case we might expect deviation from this ideal behavior for two reasons. First, the Rydberg electron for these states might be expected to significantly affect the geometry and force constants of the ion core. Since this is the lowest Rydberg, there can be mixing with other valence states. Second, we can expect strong configuration interaction between Rydberg series converging to the two spin-orbit component limits so that a single-configuration description is inadequate. This expectation follows from the existence of strong autoionization observed5 in ( 5 ) D.
M. Mintz and T. Baer, J . Chem. Phys., 65, 2407 (1976)
i
e
MICROSECONDS
1.28
Figure 3. MPI-PE TOF spectrum of methyl iodide ionized at (a) 369.94 and (b) 362.58 nm. Photoelectron kinetic energy values for the peaks are as follows: a, 0.52; b, 0.35; c, 0.72; d, 0.56; and e, 0.4 eV.
the spectral region between the two limits and caused by this same configuration interaction. To begin our study, we obtained the MPI spectrum of both C H J and CDJ in a molecular beam (see Figure 2). Significant isotope shifts are observed in the deuterated spectrum. This is expected because the active mode in the resonant states of interest is u2, the symmetric methyl wag. Peak positions and assignments for both spectra are given in Table I. From these spectra we were able to select the appropriate ionization laser frequencies for the two-laser multiphoton ionization-photodissociation experiments described below. Formation of the Methyl Iodide Parent Ion by One Laser The kinetic energy distribution of the electrons formed during ionization can be determined by multiphoton ionization-photoelectron (MPI-PE) spectroscopy. The apparatus used has been reported previously.6 This information can be used to determine
276 The Journal of Physical Chemistry, Vol. 90, No. 2, 1986
Woodward et al. ( 0 ,o,o)
I I
-
!
-
( 0,l. n)
7
8
IO
9
rn
t
II
>
t rn
2 W
OnO
M I C R O S E C O N OS
1.28
Figure 4. MPI-PE TOF spectrum of methyl iodide ionized at (a) 365.87 nm and (b) 358.58 nm. Photoelectron kinetic energy values for the peaks are as follows: a, 0.63; b, 0.48; c, 0.32;d, 0.8; e, 0.64; f, 0.49; and g, 0.31 eV .
the state of the ion being formed at a particular excitation wavelength. MPI-PE time-of-flight (TOF) spectra for the four resonant levels are shown in Figures 3 and 4. The energy value for each peak is also given in the figure captions. The error in the energy calibration is fO.l eV. Table I1 summarizes the results and gives the states of the ion that are excited for excitation through each intermediate resonance. All four resonant levels create ions in the 2113,2ground state; however, no evidence is seen for the formation of the 211!,2 state. The photoelectron energy corresponding to the formation of the vibrationless 2111,2state is 0.003 eV at 365.87 nm and 0.2 eV at 358.84 nm (but at 358.84 nm the probability is expected to be higher for the formation of ground state with one quanta of v2 at 0.095 eV). Small the 2nlj2 stray fields can make electrons with very low kinetic energies difficult to observe in a T O F spectrometer. However, if the repeller voltage is increased, the collection efficiency for the low-energy electrons is preferentially increased over that of the other electrons. Nevertheless, even under high repeller conditions, no low-energy electrons were observed. As will be discussed later, we have other evidence to show that we do form ions in the 211,,2 ground state. An explanation for the absence of the corresponding photoelectron in the PE spectrum can be found in our studies of the photoelectron spectrum of methyl iodide as a function of laser power and sample pressure.6 It was observed that electrons of less than 0.3 eV were efficiently captured to form transient CHJ ions which are then photodetached before, during, and after dissociation to CH3 I- products. This may explain why, in all four photoelectron spectra taken, the progressions in v2 always end at -0.3 eV with the next member of the progression not observed. Under these conditions it then seems reasonable that electrons produced by the formation of the 'IIIl2 ground state ion are not observed.
+
Two-Laser Mass Spectrum The apparatus used to obtain these data is essentially identical with that reported in ref 1. An excimer laser drives two dye lasers to generate the pump-probe two-laser T O F mass spectra. The T O F mass spectrum produced by the ionization laser alone has moderately intense mass 15 (CH3+)and mass 127 (I+) peaks in addition to the very intense CH31+ parent ion peak.' This frag( 6 ) W. A. Chupka, A. M. Woodward, S . D. Colson, and M. G. White. J . Chem. Phys., 82, 4880 (1985).
~
k-
z
-
:
. . I .
460.00
. . . . . . . . . . . . . . . . . . . . . WAVELENGTH
(nm)
-:..
: .
4
436.31
Figure 6. Photodissociation excitation spectrum of CH31f prepared at an ionizing wavelength of 362.59 nm (transition b in Figure 1 ) .
mentation is very probably due to photodissociation of the parent ion by the ionizing laser. It is not very extensive in this case because the ion absorption is fairly weak as may be inferred from the photoelectron spectrum. We wished to obtain the ion absorption spectrum as a photodissociation excitation spectrum by monitoring the intensity of a fragment ion peak as a function of excitation wavelength. Thus, the existing mass 15 peak constitutes a troublesome background signal which makes it difficult to observe further production of CH3+by photodissociation of CH31+. In order to eliminate this background, the dissociation laser was delayed by 60 ns which caused the CH3+produced by the photodissociationlaser to appear as a well-separated peak in the mass spectrum.' Ion photodissociation spectra were taken by using one of the four resonant levels in Figure 1 for ionization and then scanning the dissociation laser while monitoring the time-delayed mass 15 intensity. The observed photodissociation spectra were found to result from one-photon excitation to the A state of the ion from the state (or states) of the parent ion formed by ionization (vide infra). The structure observed can all be assigned to various progressions in u3. Assignments are given in the form (ul,v2,v3) which represents the totally symmetric modes. These are the only modes that can be observed for the transitions studied. Photodissociation spectra of CHJ' originating from the 2r13/2 ground state were taken following ionization at 369.94 and 362.59 nm (Figures 5 and 6). Ionization at 365.87 nm gave a spectrum similar in structure to ionizing at 369.94 nm, as was expected from the PES data, but the signal was much weaker. The photodissociation spectrum of CD31+produced with 369.24-nm radiation was also taken (Figure 7). Three photons of 369.24-nm radiation are below the 2rI,,2. ionization potential for CD31and can therefore only produce the ion in its 2113/2ground state. Photodissociation spectra of CH31+originating from the 2n,/2 ground state were taken with the ionization laser at 365.87 and 358.84 nm (Figures 8 and 9). No signal was observed when ionizing at 369.94 and 362.59 nm. Further attempts to take the CD31+spectra with the ionization laser at 365.02 nm, which could
A-R
The Journal of Physical Chemistry, Vol. 90, No. 2, 1986 277
Photodissociation Spectrum of CH31+
( o,o, 0 1D.
Figure 7.
(0, Osn)
15
14
13
Photodissociation excitation spectrum of CD,I+ prepared at
an ionizing wavelength of 369.24 nm (corresponding to transition a in
Figure 1).
(O,O,O)-( 0 . l . n )
8
7
(o.o.o)+ (0,2,n)
3
4
v)
I-
-
Lo
z W
TABLE I11 CHJt
A-g(zI13/z)PhotodissociationExcitation Spectrum of
ionization photodissociationa wavelength, nm Xair, nm uyaC,cm-l 369.94 456.64 21893 453.90 22025 451.51 22 142 448.42 22294 446.48 22391 443.10 22562 441.65 22636 438.02 22823 437.06 22874 362.59 457.52 21 851 455.48 21949 453.88 22026 452.77 22080 451.48 22 144 449.97 22218 448.40 22296 448.27 22302 446.46 22392 444.58 22483 443.96 22518 443.13 22560 441.68 22634 439.82 22730 439.55 22744 438.03 22823 437.06 22874
obsd calcdb 1.4 0.5 -2.3 -1.4 -1.2 0.3 0.9 -0.2 1.1 -0.7 0.9 1.5
0.7 -0.4 -0.1 0.6 0.4 -0.2 -0.4 -0.6 -1.7 -1.1 -0.2
assignt qU,,D2,U3) .+
-
A(ul,v2~u3)
(O,O,O) (O,O,O) (O,O,O) (O,O,O) (O,O,O) (O,O,O) (O,O,O) (O,O,O) (O,O,O)
(O,l,O) (0,1,0) (O,O,O)
(0,1,0) (O,O,O)
(0,1,0) (O,O,O)
(0,1,0) (O,O,O)
(0,1,0) (0,1,0) (O,O,O) (O,O,O)
-0.2
(0,1,0) (0,1,0)
-0.2 1.1
(O,O,O) (O,O,O)
+
-+
---+
+
-+
--+
+
-
--
+ +
+
+
(0,0,11) (0,1,7) (0,0,12) (0,1,8) (0,0,13) (0,1,9) (0,0,14) (O,l,lO) (0,0,15) (0,0,16) (0,2,7) (0,1,7) (0,0,17) (0,0,12) (0,2,8) (0,1,8) (0,0,18) (0,0,13) (0,2,9) (0,0,19) (0,1,9) (0,0,14) (0,0,20) (0,2,10) (O,l,lO) (O,O, 15)
OThere were some calibration and printing errors in the portion of these spectra reported in Table 1 of ref 4. bobserved minus calculated frequencies when the constants in Table VI labeled n are used.
A study of the photodissociation of CH31+,in which electron impact was used for ion preparation, has been done by Goss et aL2 They reported a preliminary vibrational analysis of their very complex, “hot” spectra which we used as a basis for the start of our own analysis. For our analysis we used the equation v = v0,o - u2/1(v2/1 - X2/)
+ u;v2/ + u3‘v3’
- (Ui)2X22/ -
(u3’)’X33’ - (uZu3’)x23’ a
1
1!
=-
~
595.88
.
.: , , , : ,
,
.
: . , . : . . . : . . . : . . .
W A V E L E N G T H (nm)
572.12
Photodissociation excitation spectrum of CH,It prepared at an ionizing wavelength of 358.84 nm (transition d in Figure 1).
Figure 9.
potentially form zl1112 CD31+, produced no signal. Vibrational Analysis Since we are u$ng photodissociation of CH31+ to obtain the spectrum of the A state, it is only possible to observe those vibrations which are above the dissociation threshold. The threshold is known to be about 2350 cm-I above the A-state origin,’,’ and therefore, the elecronic band origin is not seen in any of our spectra. The vibrational analysis is therefore complicated by the need for a long extrapolation to the vibrationless origin in order to obtain the vibrational constants. The fact that we see no photodissociation spectra below the one-photon dissociation threshold shows that the observed dissociation results from absorption of a single photon. (7) L. Karlsson, R. Jadrny, L. Mattsson, F. T. Chau, and K. Siegbahn, Phys. Scr., 16, 225 (1977).
+ (U3’I3y333’
where vo,o is the band origin, vZI/ - X2” is the anharmonic v2 frequency for the ground state of the ion, vZ’ and v3’ are excited A-state harmonic frequencies, X22/is the anharmonicity in vz’, X33’and Y333’ are the second and third-order anharmonic terms for v3’ and Xz3’ results from the anharmonic coupling of v2’ and v3). A last-squares program that uses the differentials with respect to each constant was used to get a consistent fit first to the 211112 initial-state data. Next, to fit the ZIIljz initial-state data with the 2113/2 initial-state data two terms were added to the equation: Aso, to account for the spin-orbit splitting between the two ground states of the ion and vi” - Xz2”’), to account for the excitation of v2 in the zI11/2state. Tables I11 and IV summarize the best simultaneous fit for the ’lI3/2 and zlIl/zdata. Unfortunately, this is not a unique fit as will be discussed later. The CD31+ photodissociation data were fitted to the CH31+photodissociation data by treating both as diatomic molecules.* The equation was modified to v = ~ o , o AuO,~- p I ~ i ’ ( ~ 2 ’ ’X22/’) u I ’ ( v ~ ’ - Xll’)
+
p1u2’v;
+
(PIU2)2x22’ - (P2’3’)2x33’
+
p2u3fv3’
+
-
- (PIuZ’P3’)X23’ + (P2u3’)3y333’
where Avo,ois the isotope shift in the zero-point energy and p I and p2 are isotope factors, defined by Herzberg,* which are less than unity. We used 0.768 and 0.94 for p1 and pz, respectively, to get the best fit. The CD31t vI’ -XI I’ frequency has been added since, with the isotope shift, we now have enough energy to excite the (8) G. Hertzberg, “Spectra of Diatomic Molecules”, Van Nostrand, Princeton, NJ, 1950, p 162.
278 The Journal of Physical Chemistry, Vol. 90, No. 2, 1986 TABLE IV: &g(zI13,2) Photodissociation Excitation Spectrum of CH# assignt ionization photodissociation obsd % W h U.J . wavelength, nm A,,, nm uyaC,cm-l calcd A(u~,u~,u~) 365.87 593.44 16 846 -0.2 (0.0.01 (0.0.111 588.76 16980 0.9 (o,o,oj (o,1,7)' 587.05 17 030 -0.7 (O,O,O) (0,2,3) 584.65 17099 0.0 (O,O,O) (0,0,12) 579.57 17249 -1.1 (O,O,O) (0,1,8) 577.23 17319 0.1 (O,O,O) (0,2,4) 576.28 17348 1.1 (O,O,O) (0,0,13) 358.84 594.54 16815 -0.3 (0,1,0) (0,0,16) 591.15 16911 -0.1 (0,1,0) (0,2,7) 586.58 17043 0.2 (0,1,0) (0,0,17) 584.66 17099 0.0 (O,O,O) (0,0,12) 581.85 17 182 0.4 (0,1,0) (0,2,8) 579.05 17 265 -0.1 (0,1,0) (0,0,18) 573.00 17447 0.0 (0,1,0) (0,2,9)
--------
-
a Observed minus calculated frequencies when the constants given in Table VI labeled n are used.
TABLE V A-g(zr13,2) Photodissociation Excitation Spectrum of CDJ' assignt ionization photodissociation obsd ~pI,u,h)
wavelength, nm A,, 369.24
nm 451.65 449.03 448.05 446.16 444.12 443.46 441.04 439.38 439.01
u,,,,
cm-'
22 135 22 264 22312 22 408 22510 22543 22667 22 753 22772
calcd"
-
A(JJI.L.>,uI)
0.1 6.7 -1.8 3.0 2.4 -2.5 -3.1 2.2 -0.3
Observed minus calculated frequencies when the constants given in Table VI labeled n are used. v1 mode. Table V summarizes the best simultaneous fit of the deuterated and protonated 2113/2 data. The fit of the deuterated data is not as good due to the simplistic nature of the diatomic model in approximating the isotope effect in a polyatomic molecule. Even with all this data a unique vibrational numbering cannot be determined. If the numbering is changed by one, a reasonable fit is still obtained. We estimate that the minimum change in the zero-point energy upon deuteration is greater than 80 cm-I. This sets a lower limit on the vibrational numbering. While an upper limit is harder to establish, we believe that our numbering is off by no more than one. Table VI gives a list of all the vibrational constants and how they change with a shift of + I to -2 in the vibrational numbering. The most significant change occurs in the band origin. The vibrational numbering used in Tables 111-V and Figures 5-9 is labeled in Table VI. It was chosen because it gave a very reasonable isotope shift and exhibited the smallest difference between the origin calculated separately for the protonated and deuterated data. It may be possible to obtain the lower levels of the A-8 transition in CH31+via laser-induced fluorescence or by using a two-photon process where the first photon would be resonant with a low-lying A(u) level and the second photon would excite the ion to a level above the dissociation threshold. Such spectra, including the band origin for both CH31+ and CD31+,would uniquely determine the vibrational numbering.
Comparison with GOSS,McGilvery, Morrison, and Smith2 Goss et aL2 give vibrational assignments for the 'IIIl2photodissociation spectrum of CH31+. They also reported having taken the spectrum for CDJ+ but provide no assignments. The vibrational numbering they choose to use corresponds to our vibrational numbering labeled n. However, the constants they report do not agree with those determined from our data for that same
Woodward et al. TABLE VI: Vibrational Constants (em-') n-2 n-1 1254 ~ 2 "- X22"(2r13,2) 1254 1245 - X22111(2r11/2) 1245 VI' - XI,' 2088 2093 1205 v11 1204 301 v3' , 297 7.4 x2 2 7.3 X33' 2.2 2.1 x23f 0.93 0.96 Y3?' -0.0097 -0.017 origin" 19404 19105 originb 19424 19123 99.7 Avo.0 85 As0 -5045 -5045 ffc 0.874 0.875 1.471 1.790 u2111
n
1254 1245 2100" 1203 305 6.6 2.1 0.84 -0.010 18806 18804 115 -5046 0.875 2.010
n+l
1254 1245 2100 1208 309 7.3 2.1 0.92 -0.010 18496 18499 130 -5045 0.875 2.093
"Obtained from fit of CH31+2r13/2and 2111/2data. *Obtained from fit of 2r13i2data for CH31+and CD31+. CStandard deviation of fit. Frequency for CD31+ion. All others are for CH31+. numbering. (They report a v2 frequency of 1141 cm-' and a v3 frequency of 265 cm-l for the CH31+A state). Due to their ion preparation by electron impact, their photodissociation spectrum is far more congested, making it more difficult to determine band positions accurately. Further, Goss et aL2 report that, due to the large number of overlapping transitions and the interference from hot bands, the spectrum from the 'II3,2 initial state cannot be assigned. Conclusion This study clearly demonstrates the advantages of using MPI to form state-selected ions for photodissociation spectroscopy. From the highly simplified photodissociation spectra obtained, we have determined the ion ground-state spin-orbit splitting and ground and excited vibrational constants with a higher precision than known previously. However, in the case of methyl iodide, ion preparation is not ideal. Two channels are available for formation of CH31+,the *II3,2 and 2111/2 states. All four intermediate vibronic levels studied are nominally of the 2J11/2core. However, we observe that if there is enough energy to create the 2113j2state but not enough to create the 2111,2state, then formation of the 2113,2 state is favored over absorption of another photon and creation of the 2111,2 state. Further, in the case of CD31+,where ionization with 365.02-nm radiation is above the *IIlj2ionization potential, only formation of 211!j2 ions is observed. The only case in which spin core conservation is observed occurs when the ionizing photon is accidently resonant with the 2J1112 ionization potential, and even then, a significant fraction of the ions are formed in the 2113,2 state. As mentioned earlier, this can be explained by the strong 2111/2-2113,2 configuration interaction evidenced by the autoionization peaks seen between these two threshold^.^ In all four cases the ions are formed with a distribution of vibrational states. The conserving vibrational state, however, always predominates. As reported previously: rotational selectrion by MPI of CH31has also proved to be less than ideal. Rotational cooling of the parent molecule is a more significant effect. This may be because the intermediate level in CH31 is prepared by pumping of a rotational band head rather than a resolved rotational line. One anomaly in the photodissociation spectra is worth noting. In the spectra arising from vibrationally hot ions, the (0,1,0) (O,l,n) transitions are not observed while both the (O,l,O) (O,O,n) and (O,l,O) (0,2,n)transitions are observed (Figures 6 and 8). This is in contrast to the strong Av2 = 0 progression seen when starting in the (O,O,O) level.
-
- -
Acknowledgment. The work of A.M.W. and S.D.C. was supported by the Army Research Office and that of M.G.W. by the U S . Department of Energy through its Office of Basic Energy Sciences. Registry No. CH31+,12538-72-6.
