J. Phys. Chem. 1995, 99, 1741-1747
1741
Photoion Alignment: Chemical Signatures 200 eV above Threshold Romith DasJ Chuanyong WuJ A. G. Mihill,$ and E. D. PoliakofPr$t*s Louisiana State University, Baton Rouge, Louisiana 70803
Kwanghsi Wang and V. McKoy Arthur Amos Noyes Laboratory of Chemical Physics,l Califomia Institute of Technology, Pasadena, Califomia 91125 Received: August 24, 1994; In Final Form: September 26, 1994@
We present results of experiment and theory for the alignment of CO+(B2Z+)and N2+(B2&+) photoions over an extended energy range (0 IE k 1. 210 eV for CO and 0 1. Ek I250 eV for N2). The polarization of CO+(B2Z+-X2Z+) and N2+(B2&+-X2Z,+) fluorescence is used to interpret the oscillator strength distributions for normally unresolved degenerate ionization channels. The results show the influence of a CO 40 ka shape resonance clearly and agreement between theory and experiment is excellent. However, agreement between the calculated and measured values is less satisfactory for N2. This behavior is somewhat surprising, as previous rotationally resolved fluorescence experiments have shown excellent agreement between theory and experiment. This comparison helps to illustrate the complementarity of alignment studies relative to alternative probes of ionization. For both N2 and CO, the data indicate that the photoions retain significant alignment even at high energies. The results demonstrate that even well above threshold the spectral dependence of the alignment $e., polarization) is very sensitive to the molecular environment for photoejection. Such behavior provides useful insight into fundamental scattering phenomena in chemical physics.
-
Introduction Molecular photoionization exhibits a rich variety of scattering phenomena and properly designed studies can generate fundamental chemical insights. Due to the complexities of its underlying behavior, attempts to disentangle molecular photoionization dynamics can benefit from complementary experimental In this paper we describe an investigation of the polarization of N2+(B2Z+-X2Z,+) and C 0 + ( B 2 F + X2Z+) fluorescence. Such studies yield information on the alignment of the molecular photoions and, thus, provide useful data on spectral signatures of the underlying c~ntinua.~-lO Photoion alignment is determined largely by the relative strengths of degenerate ionization continuum channels of alternative electronic ~ymmetry?-~J~ and hence alignment data naturally reflect the symmetry of the molecular target. As a result, alignment data are useful complements to photoelectron cross sections and asymmetry parameters.”J2 An additional benefit of alignment studies which has not been exploited previously is that they can be performed over a very broad range, thus providing a global perspective on the photoelectron dynamics. Indeed, the extended energy range of the present study (20 5 hv,,, 5 270 eV) further demonstrates that such a perspective is particularly illuminating. Recent synchrotronbased investigations into molecular ionization dynamics demonstrate that the broad spectral coverage afforded by such sources reveals molecular aspects of photoelectron behavior with greater clarity than studies clustered near the ionization threshold.13-16 The key point of the present polarization study is that the escape of the photoelectron determines the alignment and that even far from threshold the alignment is very sensitive
* Corresponding author.
* Department of Physics. * Center for Advanced Microstructures and Devices. P Department of Chemistry.
Contribution No. 8977. @Abstractpublished in Advance ACS Abstracts, January 15, 1995.
0022-365419512099- 1741$09.0010
to the short-range aspects of the molecular potential. Indeed, the comparison for N2 2a,-’ and CO 4a-’photoionization will show strong differences in the broad range studied. Molecular photoionization is intrinsically degenerate, Le., a photoelectron always has at least two continuum channels in which it can exit. For example, an electron in a bound CJ orbital can be ejected into either a ka or a Ivc electronic continuum. The relative strengths of the alternative continuum channels is highly energy dependent and traditional measurements do not readily provide information on their respective contributions to the total oscillator strength. For example, the partial photoionization cross section measures the incoherent sum of altemative channels, while the photoelectron asymmetry parameter is strongly influenced by the relative phases of the partial waves. It has been recognized for some time that measurements of the photoion alignment can provide pertinent information on the relative contributions of alternative ~hannels.~-llMany studies have exploited fluorescence polarization from electronically excited photoions as a means of generating such alignment data. In fact, such studies have generated a useful counterpoint to traditional photoionization spectroscopies, such as photoionization mass spectroscopy, electron spectroscopy, and vacuum ultraviolet absorption spectroscopy. While previous investigations demonstrated the benefit of fluorescence polarization, they have always been constrained by the limited spectral range studied. The present results demonstrate the benefits that accrue from extending studies deep into the ionization continuum. In a preliminary communication on the CO data,15we stressed a single aspect of the results presented here. We showed that the fluorescence polarization did not reach a limiting value at energies more than 210 eV beyond threshold and that there was no indication that the alignment approached zero at higher photon energies. The present study attempts to determine how the molecular “fingerprints”, as manifested in polarization data, might be of use in interpreting the photoelectron dynamics. The ability to investigate the ionization dynamics over hundreds of 0 1995 American Chemical Society
1742 J. Phys. Chem., Vol. 99, No. 6,1995
Das et al.
Figure 1. Experimental schematic.
electronvolts has also been exploited in recent rotationally resolved dispersed fluorescence studies of photoionization. The present fluorescence polarization study is part of this broader program which provides a spectral overview of the ionization dynamics. The present results permit several useful comparisons. First, we can compare fluorescence polarization results for N2 and CO. Second, we can evaluate the comparison between theory and experiment for these polarization results. Third, we can study the differences between the rotationally resolved fluorescence data and the fluorescence polarization data for these systems. These comparisons all demonstrate how alignment data provide a complementary probe for disentangling the dynamics of complex ionization events. Moreover, they demonstrate that fluorescence polarization studies with wide spectral coverage provide useful guideposts for developing insights into fundamental ionization dynamics which are otherwise inaccessible.
Experimental Section The experimental procedure has been described previously" and is reviewed briefly here. The experimental geometry is shown in Figure 1. The electronic configuration of N2 is K K ( ~ U ~ ) ~ 1nu)4(3ug)2, ( ~ U ~ ) ~ (and we study the 2uU-' electron ejection in this study. The excitation-fluorescence sequence for N2 is
+
Nz(XIZg+) hv,,,
-
N2+(B2&+)
+ e-(kug,kng) N>(X2Z,,f) + hv,
(1)
For CO, the electronic configuration is K K ( 3 ~ ) ~ ( 4 ulsc>4(5u)2, )~( and the 4u-I ejection is investigated. The sequence for CO is analogous, the only difference being that the g/u characterization relevant to inversion symmetry is absent. The source for the soft X-ray excitation radiation is the synchrotron radiation facility at Louisiana State University, the Center for Advanced Microstructures and Devices (CAMD)." All of the measure-
ments described here used the 6-m plane grating monochromator (PGM) beam line.lS The excitation monochromator is operated with the premirror optimized for high throughput,ls and the lowenergy grating (360 grooves/mm) is used for the entire excitation energy range. This wide spectral coverage is possible without any grating changes, which is a significant advantage for survey studies, such as those described here. We use an A1 filter to evaluate influences of higher-order contributions on the measured polarization above 36 eV. No effects beyond experimental data scatter are measurable. The excitation monochromator is operated with 2 mm slit heights (entrance and exit), resulting in an energy bandwidth AE 0.5 eV. The degree of linear polarization of the incident radiation is estimated to be approximately 95% near threshold, dropping to 75% at hvexc= 270 eV. The results shown below are corrected for the incomplete polarization of the incident radiation. A two-stage capillary differential pumping system guides the excitation radiation to the interaction region (&gh = 6 x Torr) and maintains the ultrahigh vacuum of the beam line (Plow 1 x Torr) and electron storage ring. This 6-order-of-magnitude reduction allows us to maintain a relatively "bright" fluorescing volume, which results in the strong signals seen in these measurements. This is particularly important as the ionization cross section is exceedingly low at 200 eV above threshold. The ionizing radiation intersects the gas sample, which emerges from an effusive nozzle. An effusive gas source is used rather than a supersonic jet in order to keep the neutral target molecules rotationally hot (250-290 K). This facilitates the analysis by enabling the high-J limiting expressions to be used." The background gas pressure in the chamber was maintained at 6 x low4Torr for the measurements presented here and we also checked the results at lower pressures to ensure that the data were free of artifacts due to secondary processes.1° We estimate that the effective pressure in the interaction region is 10-100 times greater than the background chamber pressure. The fluorescence optics are sketched in Figure 1. The optical layout is different from the one used in previous studies because the CO+(BZZ+-X2Z+) fluorescence is in the ultraviolet (Afl= 2200 A), necessitating the use of a crystal polarizer (Karl Lambrecht Model TFPC-12). A sheet polarizer was used to analyze the polarization of the N2+(B2&++XZZg+) fluorescence. An interference filter was used to reject the CO+(A211-X2Z+) fluorescence as well as atomic and atomic ion fluorescence resulting from photodissociation processes. All of the data acquisitions links between the control computer and the polarization analyzer, the excitation monochromator, and the vacuum ultraviolet photodiode were controlled via CAMAC and serial links, similar to data acquisition procedures described previo~sly.4~~~~
Theory Theoretical analysis and associated calculations are particularly important for this study as previous experiments have not been performed at such high energies. The high photoelectron kinetic energies can result in substantial rotational excitation, which in turn can influence the molecular alig~ment.~-~%" Here, we briefly review the theory for fluorescence polarization from molecular photoions.ll As eq 1 indicates, an electron ejected from the NZ2u, orbital can exit via the kug and kng continuum channels. Ejection through the kug channel results in an electron-ion complex that has &+final-state symmetry, while ejection into the kng continuum produces an electron-ion complex with nu symmetry. The key point is that the absorption transition dipole tilts relative to the plane of rotation depending on the relative strengths and interference between
J. Phys. Chem., Vol. 99, No. 6, 1995 1743
Photoion Alignment partial waves of the competing channels, thereby modulating the degree of alignment!, Classically, the alignment is a measure of the spatial anisotropy of angular momentum vectors, while quantum mechanically it is a measure of the unequal populations of MJ sublevels. In the present case, we are interested in the alignment for the J+ levels of the excited B states of N2+ and CO+. The alignment, Ao(~)(J+), ranges from 2 to -111919920and is determined by
where a(J+,Mp)is the total cross section for the M p sublevel of the J+ level. In our calculations, a(J+,Mp) is summed over all of the contributions from the MJ, sublevels of the initial JO levels:
values of N+ included in this averaging provided N+ is not too close to zero. To contrast this analysis with previous ones performed at lower photon energies," we calculate the alignment of the rotational levels quantum mechanically and use only the highN+ levels to predict the polarization. Previous treatments at lower energy (e.g., ref 9) assume implicitly that the alignment can be determined semiclassically, which is not appropriate for the high energies employed here. It is interesting to compare the fully quantum mechanical results with semiclassical results which assume that the plane of rotation is not significantly affected by the ionization process and that interference terms among partial waves do not influence the polari~ation.4~~ The dipole strengths are defined as Doz = ZllDI;)l2 and DZ2 = 2 ~ l D ~ ~ ~ and * l 1the 2 , ratio R defines the relative strengths, Le., R = Dn2/Do2 . For a pure a k a excitation, the absorption transition is Z Z and the transition dipole lies parallel to the internuclear axisz3 One can show that this pathway results in = -2/5 and P = +l/7. On the other hand, a pure a kn excitation is a Z ll transition and the absorption dipole lies perpendicular to the internuclear axis. In this case, (AO(~)) = +'/5 and P = -'/13. For a situation which is neither pure k o nor pure kn, there is a relation between the dipole strength ratio R and
-
-
-
+
where QM is the population of the Mj0 sublevel of the initial state and" tfie coefficients C~,(MJ,,M~)are related to the probability for photoionization of the magnetic MJ, sublevel leading to the M p sublevel of the ion. For a 2-2 transition of interest here, both initial and ionic states can be best described by the Hund's case (b) coupling scheme and Clm(M~o,Mp) has the simple fonn21,z2
R=
2
+ 5(A,'2')
1 - 5(AT')
which assumes that the plane of rotation does not change during the excitation or fluorescence p r o ~ e s s .Equation ~ 6 is commonly used to relate the fluorescence polarization to the relative strengths of the degenerate pathway^.^^^ Specifically, eqs 5 and 6 can be combined to obtain the relationship between R and P :
R = -2 - 14P 1 13P
+
where C' are some laboratory-frame quantities, Dap(-) the photoelectron matrix element between the initial state and the photoelectron wave function, Nt the angular momentum transfer, m and ;1 the projections of 1 in the laboratory and molecular frames, respectively, and p and po the photon polarization index in the molecular and laboratory frames, respectively. In our calculations of the alignment parameter Ao(~)(J+), we assume an initial rotational temperature of 250 K for NZ and CO. The analysis here is quantum mechanical, as opposed to the semiclassical treatment used for previous ~tudies.~JOTo convert the results into polarization data appropriate for comparison to experiment, we use a simplifying approximation. Specifically, because the major contributions to the fluorescence will be from high-J+ levels (i.e., the target molecules are warm), it is possible to use a limiting expression which relates the polarization to the alignment for Z Z fluorescence," i.e.,
-
(AT') = -8P/(3 - P )
(5)
where (AO'~)) denotes a high-J thermal average. To calculate the polarization P of eq 5, (AO(~)) is determined by averaging over all Ao@)(J+)with N+ 2 10. Here N+ is the total angular momentum exclusive of spin for the B state of CO+ or N2+ and J+= N+ f l/2. Such an average is appropriate since eq 5 is valid only in the high-J limit and the Ao(*)(J+) are approximately the same for both J+= N+ '/2 and J+= N+ - '/2 levels for high N+. is not sensitive to the actual
+
(7)
Equation 7 is predicated on the assumption that the photoelectron dynamics do not influence the molecular rotation significantly. This simplification is not true at the higher photon energies, as the change in angular momentum (AN = N+ - NO)is not small compared to the target angular momentum at the higher photon energies.l4 Final-state photoelectron wave functions were generated for the 2uu ko,, kng channels of N2 and for the 4 0 ka,kn channels of CO. The calculations are performed at the Har~reeFock level and the photoelectron orbitals are obtained by numerical solution of the Lippmann-Schwinger equations using an iterative procedure, based on the Schwinger variational principle.24 Details of calculations can be obtained from refs 14 and 15.
-
-
Results The calculated magnitudes of the photoelectron matrix element for CO and N2 are given in Figures 2 and 3, respectively. The magnitudes, IDlii(-)(, are given for partial waves in both continuum channels (i = 0 for ka and i = f 1 for kn). The composition of these matrix elements shows that the photoionization dynamics is far from atomiclike, even at the highest energies. To see this, consider the composition of the Nz 20, orbital, which is 90.65% p and 6.66% f character. With the target orbital being of p character, dominant s (I = 0) and d (I = 2) partial wave contributions to the photoelectron matrix element would be expected on the basis of an atomiclike propensity rule. This is not observed for N2 or for CO. Both
Das et al.
1744 J. Phys. Chem., Vol. 99, No. 6, 1995 1.0 -
CQ 4u -b ku
N,
+1=0 rc
-d
0.5
+1=2 +1=4
1=2
s
0.4.
-
0.3 .
n
8 1=3
d v
t 1=4
0.5
+1=0
-
I
41 = 1
78
-0
I O
n
V"
V
n -
2uu -b ko,
-5 1 = 8
I
-
0.2
-
0.1 0.0 1.0
0.0 1
-
CQ 4u -b kn
+1=1 rc
N, 2uu -b krr,
0.8 -
+1=2
1=2
81=3
h
-c t = 4
+- 1 = 5
0.1
0.0 20
70
120
170
PHOTON ENERGY (eV1 Figure 2. Calculated magnitudes of dipole strengths for individual partial waves for 4u-' photoionization of CO.
molecules show the growth of higher angular momentum components with increasing energy, as expected. Figure 2 shows a strong shape resonance in the 1 = 3 component of the 4 0 k a channel of CO at hv,,, RZ 35 eV. The NZ results in Figure 3 do not show an analogous shape resonance, as only even 1 values are possible for the 2a,-' channel. However, Cooper minima are observed in the 1 = 2 and 1 = 4 components of the ku, continuum of N2. The 1 = 2 component exhibits a Cooper minimum at hv,,, x 140 eV. The 1 = 4 component has two Cooper minima, a weak one at hvexc x 50 eV and a more pronounced one at hv,,, RZ 100 eV. These Cooper minima are explicitly seen in the standing-wave form of the photoelectron matrix element, No Cooper minima are observed for CO. The polarization results for CO and Nz are shown in Figures 4 and 5, respectively. The bottom frame of each figure shows the measured polarization data along with predictions from calculations. The top frame shows the ratio of dipole strengths, Le., the R parameter of eq 7 . In each bottom frame, the fully quantum mechanical predictions are shown as a dashed line and the semiclassical predictions based on the ratio of dipole strengths are shown as a solid line. Surprisingly, the qualitative trends observed in the experiments are seen to be reproduced faithfully by eq 7, which was not intended for use too far from threshold. While qualitative insights can be obtained by exploiting eq 7, quantitative interpretations require the use of eqs 1-5, particularly at the higher photon energies. For CO, the experimental results are in good agreement with the fully quantum mechanical theory of eqs 1-5 and in fair agreement with the semiclassical model of eqs 6 and 7. Moreover, the experimental results for CO are in excellent agreement with results obtained previously over a more limited spectral range.5
-
25
220
75
125
175
225
275
PHOTON ENERGY lev)
Figure 3. Calculated magnitudes of dipole strengths for individual partial waves for 2uU-' photoionization of Nz.
N
s
1.00
a:
0.00 0.10
0.08
-B h
a
5E
0.06
0.04 0.02
a
20
70
120
170
220
PHOTON ENERGY (eV)
Figure 4. C O + ( B W d X Z F ) fluorescence polarization results. The bottom frame displays measured fluorescence polarization data and predictions from theory. Dashed line: quantum mechanical results from eq 2. Solid line: semiclassical results from eq 7. The top frame gives the calculated ratio of dipole strengths.
For Nz,agreement between calculated and measured polarization fluorescence is less satisfactory especially near threshold. The experimental data are not suspect, as the agreement between previous and current experiments is although there is a slight disagreement near threshold. Previous determinations gave P a 0.058 while this study gives P x 0.04. It is likely that the previous values are more accurate, as higher order
J. Phys. Chem., Vol. 99, No. 6, 1995 1745
Photoion Alignment
.L h
a
i
0.10
v
8 0.05
1 0
a
0.00 -I\ 20
70
120 170 220 PHOTON ENERGY (eV)
270
Figure 5. NZ+(B~X,+-X~Z,+)fluorescence polarization results. The
bottom frame displays measured fluorescence polarization data and predictions from theory. Dashed line: quantum mechanical results from eq 2. Solid line: semiclassical results from eq 7. The top frame gives the calculated ratio of dipole strengths. contributions that are more prevalent in the current study will tend to reduce the measured values. For qualitative trends of interest here, this discrepancy is minor and we will not consider it further. The theoretical results also appear to be free from systematic calculational errors. Indeed, the region near threshold was observed to exhibit similar disagreement with theory in earlier multiple scattering calculations,26~27 suggesting that the disagreement is a result of peculiarities of the N2 ionization dynamics near threshold rather than any experimental or calculational pitfalls.
Discussion
-
First, we consider the effect of the CO 4u ku shape resonance exhibited in the 1 = 3 component (Figure 2) on the polarization profile. Indeed, this feature at hv,,, w 35 eV is clearly evident in the polarization profile in Figure 4. This resonance has been observedz8 and identified29 in previous studies of partial photoionization cross sections. A comparison of the 4a k a 1 = 3 component shown in Figure 2 with the polarization peak in Figure 4 shows that the polarization is simply reflecting the enhanced oscillator strength in the ku continuum. This CO case is the Fist observation where a polarization profile tracks the evolution of the shape-resonant oscillator strength. Note that the shape resonance influences the polarization over a range from 20 to 60 eV, emphasizing the utility of surveying the photoionization trends over ranges comparable to features of interest. There is no analogous feature in the N2 data, as no features result in a buildup of oscillator strength in either 2u,-' continuum. The data for CO and NZboth exhibit molecular behavior even at high energy. The magnitude of the polarization does not approach zero at energies beyond 200 eV.15 The ionization potentials for the CO+(B2C+)and N2+(B2&+) states are 19.7 and 18.8 eV, r e s p e ~ t i v e l y . ~Thus, ~ even at photoelectron energies that exceed the binding energy by more than an order of magnitude, the ions are produced with considerable alignment." The observation that the polarization is positive indicates that the photoelectron has a greater tendency to be ejected via the k a continuum than the Ivc channel (Le., parallel to the internuclear axis rather than perpendicular to it).15 At
-
I
I
-0.30 0
5
20
10
25
30
Figure 6. Alignment parameter A&J +) for photoionization of the 4a orbital of CO at several photon energies. Only rotational levels with J+ = N+ '12 are shown.
+
high energies, theory and experiment agree well for Nz, even though agreement is poor at lower energies. Also note that at the high energies, the fluorescence polarization values for CO and N2 are different. The polarizations at the highest energies measured for CO and NZ are P w 0.045 and P w 0.028, -0.12 for respectively. From eq 5 this translates to CO and ( A o ( ~e ) ) -0.07 for Nz. In other words, not only is there no limiting behavior, as was pointed out p r e v i ~ u s l y , ' ~ ~ ' ~ but the long range behavior is chemically specific. This study is the first to provide the basis for qualitative and quantitative tests at extremely high photoelectron kinetic energies. As a result, these data have implications for work beyond the immediate study. For example, EXAFS oscillations result from fine details of the photoelectron wave function at high kinetic energie~.~'The analysis of EXAFS data generally assumes a simple form for the photoelectron wave function that emphasizes atomiclike character of the photoelectron wave function. The present N2 and CO results far from threshold can be used for testing the validity of such assumptions. To gain further insight into the dynamical aspects of the photoionization, we show the alignment parameter A o ( ~ ) ( J at +) several photon energies for CO in Figure 6. These energies (i.e., 32.7, 78.2, 149.7, and 199.7 eV) are chosen to best represent the entire spectral range. In Figure 6, only one of * / 2 ) is shown since the the J+ components (Le., J+ = N+ other component has almost the same A#)(J+) values except at low J+ levels. Surprisingly, the alignment parameters for all photon energies decrease monotonically with increasing rotational quantum number. This implies a preference for populating the IMJ+I e 0 sublevels over the IMJ+x J+ sublevels at higher rotational levels. This behavior is also seen at other photon energies (not shown). This result is quite interesting since the initial JO levels are unaligned (Le., all MJ, sublevels are equally populated) and the same photoelectron matrix elements are used in our calculations for every MJ+sublevel. This trend can be easily understood from eq 4 through the angular momentum couplings represented by the first two 3 - j symbols. From the symmetry properties of 3 - j symbols, the change in angular momentum projection is confined to
+
Das et al.
1746 J. Phys. Chem., Vol. 99, No. 6,1995
EXPERIMENT THEORY
Figure 7. Recent rotationally resolved photoion distributions from experiment and theory on N1 and CO systems.14
MAT+- M N = ~ -m. Since the dominant partial wave components of the photoelectron have small angular momenta (1 5 4) and the initial state population peaks around JO * 10, IM,++I x 0 (IMJ+* 0) sublevels will yield larger cross sections than those with /MAT+/ fl (IMP/ NN J+) sublevels for large N+ (J+) levels. Other interesting features include the following: (i) most J+ levels have alignment parameters A o ( ~ ) ( J +5) 0 except for those at a photon energy of 78.2 eV. At 78.2 eV, the Ao(*)(J+) are positive at lower J+ levels and negative at higher J+ levels. This reflects the valley in the behavior of the polarization of Figure 4 between 50 and 110 eV; (ii) the 1 = 3 shape resonance pushes the A o ( ~ ) ( J +to ) relatively small values even at lower rotational levels; (iii) the contributions to the fluorescence polarization from the highest rotational J+ levels are small since they have relatively small total cross sections. These observations also hold in the case of N2 except for the shape resonance behavior. Often, the claim is made that alternative techniques provide complementary insights into a problem (e.g., refs 1- 5 ) , but it is rarely possible to demonstrate this concretely. The present results give us an unusual opportunity to do this quite graphically. Specifically, we contrast the polarization results (Figures 4 and 5 ) with data from a recent rotationally resolved photoionization inve~tigation.'~,'~ The results from the previous rotationally resolved study are given in Figure 7. Rotational excitation in photoionization is due to the Z-changing collisions that occur as the photoelectron escapes. Thus, the evolution of padcular partial waves can cause dramatic changes in the degree of rotational e~citation.'~.'~ This can be contrasted with the polarization experiments, where the behavior is dictated primarily by the total oscillator strength in either the k o (ko,) or the kn (kn,)continuum.
We now compare the polarization data with the rotationally resolved data phenomenologically to illustrate that the information content of the two types of measurements are indeed complementary. First, Figure 7 shows that the N2 rotationally resolved experiments agree beautifully with theory; in Figure 5 , the polarization experimental data disagree with experiment unambiguously near threshold. Second, Figure 7 shows that the N2 rotationally resolved data display a pronounced inflection point at the Cooper minimum in the 1 = 4 channel around 100 eV; in Figure 5, the polarization experimental data are featureless at hv,,, = 100 eV. Third, Figure 7 shows that the CO curves are comparatively flat in the hvexc 35 eV region; in Figure 4, the CO polarization data show a dramatic resonant peak. Indeed, these and many other striking differences are a convincing demonstration that polarization data do indeed provide a useful independent probe of the photoionization dynamics. Finally, it is useful to address the disagreement between the measured and calculated values for N2, which is striking. The theory gives us insight into the cause of this discrepancy. The reason that the theory exhibits P +'/7 near threshold is because the oscillator strength in the hg continuum is predicted to be vanishingly small (see Figure 3). This is an underestimate for the kn, contribution, which has been identified previ0usly,4,~J~ although not explained in sufficient detail. It is likely that intrachannel redistribution of oscillator strength is responsible for the observed behavior but the question still remains as to why NZ is affected so strongly while CO is not. This ability to use alternative techniques to pinpoint areas of agreement and disagreement is quite powerful and is a central message of the present fluorescence polarization study. In closing, we summarize our principal conclusions. First, the ability to scan a broad range makes it possible to examine the trends in the ionization dynamics in a global context.
Photoion Alignment Indeed, the comparison of the N2 and CO data over such a broad range is valuable and the data show that chemical aspects of the scattering emerge even hundreds of electronvolts above the ionization threshold. Second, the information content of the polarization measurements is different from, and complementary to, the data available from alternative measurements. The comparison with rotationally resolved photoionization studies l4 reinforces this point strongly. Third, these results underscore a simple physical argument, namely, that molecular aspects of the ionization dynamics emerge most clearly when molecular properties are interrogated. Indeed, this study is part of a broader program that includes determinations of different types of molecular motion following photoionization, including vibration, rotati0n,l4 and alignment.15 The ability to probe molecular aspects of the ionization dynamics far from threshold provides a useful perspective into fundamental scattering phenomena in molecular and chemical physics.
Acknowledgment. The efforts of the CAMD staff are greatly appreciated and we are particularly indebted to Drs. Vollcer Saile, John Scott, and Eizi Morikawa for their support. E.D.P. acknowledges support from NSF (CHE-9315857) and the Louisiana LEQSF program. Work at the California Institute of Technology was supported by the Air Force Office of Scientific Research and the Office of Health and Environmental Research of the US.Department of Energy. We also acknowledge use of resources of the Jet Propulsion Laboratory/Caltech Cray Y-MP2EI232 supercomputer. References and Notes (1) Nenner, I.; Beswick, A. In Handbook on Synchrotron Radiation; Marr, G. V., Ed.; North-Holland: Amsterdam, 1987; Vol. 11, Chapter 6. (2) Dehmer, J. L.; Parr, A. C.; Southworth, S. H. In Handbook on Synchrotron Radiation; Marr, G. V., Ed.; North-Holland Amsterdam, 1987; Vol. 11, Chapter 5 . (3) Gallagher, J. W.; Brion, C. E.; Samson, J. A. R.; Langhoff, P. W. J. Chem. Phys. Rej Data 1988, 17, 9. (4) Poliakoff, E. D.; Dehmer, J. L.; Dill, D.; Parr, A. C.; Jackson, K. H.; Zare, R. N. Phys. Rev. Lett. 1981, 46, 907. (5) Guest, J. A.; Jackson, K. H.; Zare, R. N. Phys. Rev. A 1983, 28, 2217. (6) Guest, J. A.; O’Halloran, M. A.; Zare, R. N. J. Chem. Phys. 1984, 81, 2689.
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