- Email: [email protected]
Vector correlations in the 308 nm photodissociation of ICN
12 September 1997
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 276 (1997) 103-109
Vector correlations in the 308 nm photodissociation of ICN Simon W. North a, Julie Mueller b, Gregory E. Hall a,* a Chemistry Department, Brookhaven National Laboratory, Upton, N Y 11973-5000, USA b Department ofChemisto', Cornell University, Ithaca, N Y 14853, USA Received 6 June 1997; in final form 6 June 1997
Abstract
Nascent Doppler profiles of CN (X 2£ +) fragments from the 308 nm photodissociation of ICN have been measured using high-resolution transient frequency modulated (FM) absorption spectroscopy. The complete set of vector correlations observable with linearly polarized single photon detection has been determined for several rotational states in v = 0. All detected states exhibit strong positive velocity anisotropy indicating that the initial optical excitation is predominantly a parallel transition. The result challenges recent theoretical work which predicts that at wavelengths > 300 nm a perpendicular transition to the 31-Ii state dominates the dissociation. © 1997 Elsevier Science B.V.
1. Introduction
The photodissociation of ICN in the A-continuum has attracted a wide range of increasingly sophisticated experimental and theoretical investigations over the past twenty years, becoming a prototype for multiple-surface dissociation dynamics. The most detailed experimental work has been performed at wavelengths close to the peak of the absorption profile, notably at 266 nm and 248 nm [1-4]. Here, two channels compete, producing iodine atoms in either their ground or spin-orbit excited states: ICN(X ']~+) + hv---~ CN(X 2 ~ + , v , N ) + I(2p3/e),I(epI/2).
(1)
The elucidation of the optical excitations and the multiple-surface dynamics leading to these asymptotic products is a long and unfinished story. Good
* Corresponding author. E-mail: [email protected].
summaries of recent work can be found in references [3] and [5]. At the red edge of the A band, corresponding to wavelengths longer than 300 nm, there has been less detailed experimental work, and the situation is apparently simpler. The energetic threshold for forming CN and excited (2Pl/2) iodine atoms is approximately 295 nm, so the dissociation has only a single asymptotic channel open [6-8]. The CN rotational distribution appears to be single component, which lies intermediate between the high and low rotational components observed at shorter wavelengths [9,10]. Despite the decreased available energy in the red wing of the absorption band, excitation here results in a somewhat higher degree of CN vibrational excitation than observed at 266 nm or 248 nm. At 308 nm Fisher et al. measured the population in v = 0 : l : 2 to be 0.88:0.10:0.02 [9], while Baronavski reported a vibrational ratio of 0.91:0.06:0.03 at 299.4 nm [11]. Recent theoretical work has provided a clearer view of ICN dissociation dynamics. Yabushita and
0009-2614/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 0 0 0 9 - 2 6 1 4 ( 9 7 ) 0 0 7 9 0 - 2
104
S. W. North et al. / Chemical Physics Letters 276 (1997) 103-109
Morokuma have calculated new ab initio excited state potential energy surfaces for ICN and have identified three dipole allowed states l IIl(E), 3I-I0+(Al) and 31-I~(E) that are all thought to contribute to the A band absorption [5,12]. Classical and quantum dynamics calculations using these surfaces or minor variations have successfully reproduced many of the experimental observations [5,13-17]. Theory predicts that the 3[I l state should dominate the absorption at longer wavelengths. Trajectory calculations performed on this lowest energy surface qualitatively reproduce both the vibrational and rotational state distributions observed at the red wing of the excitation spectrum. Griffiths and E1-Sayed have recently measured I(2p3/2) fragments arising from ICN photodissociation at 304 nm by one-dimensional photofragment translational spectroscopy [18]. The spatial anisotropy of the fragments suggested that the dissociation initially involved a predominately parallel transition. They have argued that a significant fraction of the absorption must therefore still be to the 3H0+ state. The purpose of the present paper is to further examine the vector properties of ICN dissociation in the long wavelength tail of the absorption spectrum, extending the work of Black et al. [3,4], to longer wavelengths. Our measurements at 308 nm represent a detailed set of observables which can provide a stringent test of the recent ab initio potential energy surfaces and transition moments. Extensive new FM Doppler measurements at shorter wavelengths have also been performed. The conclusions differ in some important ways from previous reports and will be the subject of a more thorough future report [19].
2. Experiment The general features of the experimental apparatus have been described previously [20-22]. Briefly, the photolysis light was provided by an unfocussed beam from an XeC1 excimer laser operating at 12 Hz. The unpolarized beam was passed through 10 quartz plates at Brewster's angle with respect to the direction of propagation resulting in ~ 95% linear polarization. Adjustment of the photolysis polarization was achieved by rotating the stack of Brewster's
~
probe
idiss
(1)
(2)
Fig. 1. Schematicdiagram illustratingthe three independentexperimental excitation-detectiongeometries. Transient Doppler measurements correspondingto geometries(1) and (2) were taken in the collinearsingle-pass cell. Data correspondingto geometry(3) were taken in the multipass transversecell. plates. The photolysis pulse provided fluences of 10-20 m J / c m 2 to the sample. The nascent (X 2~) CN radical photoproducts were probed in the Av = 2 bands of the A 2 I ] ~-- X 2~ system near 800 nm using the continuous light from a linearly polarized Ti:sapphire ring laser. The transient FM method for Doppler spectroscopy is a zero background, low noise technique for one-photon transient absorption spectroscopy. Full details have been published [21,22]. Neither the single mode ring laser bandwidth nor power broadening makes a significant contribution to the observed Doppler spectra. The use of the A - X system, with Q branches and fully resolved fine structure levels, allows the acquisition of a more complete and less blended set of Doppler lines for a more stable analysis than is possible with the more commonly used B - X system, as noted previously [3]. Three independent excitation-detection geometries were employed in the experiment and are shown schematically in Fig. 1. Measurements in geometries 1 and 2 were performed in a 1.5 m single pass collinear-cell in which the probe and photolysis beams counter propagate. Measurements in geometry 3 were performed in a transverse multipass-cell. In this cell, the frequency modulated probe laser beam made multiple reflections between plane-parallel dielectric coated mirrors aligned parallel to the photolysis laser beam propagation direction [23]. In both arrangements a slow flow (4 sccm) of room temperature ICN (Aldrich) was maintained at a total pressure of 20 mTorr. Inspection of the time-dependence of the Doppler profiles indicated that negligible transla-
S. W. North et aL / Chemical Physics Letters 276 (1997) 103-109
tional or rotational relaxation had occurred in the first 100 ns.
105
convolution over the thermal parent velocity distribution [19]. The result is that the observed laboratory Doppler profiles, D'ob~(w),can be written as a linear combination of three basis functions,
3. Results
D'obs(W) = Several rotational states of CN (v = 0, N) were measured with Q and R branch transitions in geometries 1, 2, and usually 3. The raw transient FM data were transformed to Doppler absorption profiles using techniques described in refs and. The Doppler profiles were analyzed using the bipolar moment formalism of Dixon [24] as applied in the case of linearly polarized one-photon absorption spectroscopy [23]. The Doppler profile for an ensemble with a single speed, v, can be written as
l[go+g2P2(-~,)+g4P4(W)l D(w) = 2--7'
(2)
where w is the component of the laboratory velocity along the probe direction and P2 and P4 are Legendre polynomials. For linearly polarized, single photon detection, the coefficients gi are given by the following expressions,
go = 1 + b I/3o2(02) go =be/3o2(20) + b3/3°(22) + b4/3O2(22)
(3)
g4 = b5/32(42) where b i are the bipolar multipliers which depend on geometrical factors, including the incomplete degree of polarization, and the rotational branch, and/3ff(k 1, k 2) are the renormalized bipolar moments of the correlated velocity and angular momentum distribution. In the presence of internal energy of the parent there is a distribution of speeds, each speed corresponding to a different initial parent internal state. It is straightforward to generalize Eq. (2) to include a thermal average over parent internal states and the
goD'o( W) + g2D'2( w) + g4D4( w). (4)
We have chosen to simultaneously fit the normalized Doppler profiles for each rotational line in three geometries with both branches to determine the full set of bipolar moments. We iteratively adjust the values of the center-of-mass bipolar moments to minimize the X 2 of the entire data set. We find that this method gives the fairest estimate of the errors and is stable with high quality data [ 19]. A sample of the measured Doppler profiles and fits is shown in Fig. 2. The fourth order term D~, usually neglected in Doppler LIF experiments, makes a significant contribution, particularly in geometry 1. We have corrected for the incomplete polarization of the photolysis laser in the data analysis. The results for fitting Q and R lines in multiple geometries are given in Table 1 for the N = 20, 30 and 40 states (F 1 components) of CN in v = 0. We find that the Doppler analysis is most sensitive to the values of/3~(20) and /30(22) as indicated by the error limits cited. Particularly surprising is the substantial deviation of the v .j correlation, /30(22), from its limiting perpendicular value of - 0 . 5 . This would be strictly forbidden by angular momentum conservation for total angular momentum zero, but can be allowed in a thermal sample of 1CN. Such deviations have been reported [3] and then retracted [4] for ICN at 248 rim, recognizing the difficulty of extracting this information from the B - X spectra, which lack Q branch lines. To illustrate the sensitivity of the data to the /3°(22) moment, Fig. 3 shows an optimized fit of all other bipolar moments to the same data set, with the v . j correlation constrained
Table 1 Bipolar moments for CN (t, = 0, N) fragments from ICN photodissociation at 308 nm
N = 20 N = 30 N = 40
/32(02)
/3~(20)
/3o(22)
/3o2(22)
/32(42)
- 0 . 2 8 ::[:0.12 - 0 . 4 0 + 0.10 - 0 . 3 8 + 0.12
0.77 + 0.03 0.70 + 0.03 0.60 + 0.04
- 0 . 2 7 ± 0.06 - 0 . 3 5 + 0.04 - 0 . 3 2 + 0.04
0.24 ± 0.06 0.29 + 0.06 0.23 + 0.06
- 0 . 3 2 _ 0.08 - 0 . 3 7 ± 0.08 - 0 . 3 9 _ 0.08
S. W. North et al. / Chemical Physics Letters 276 (1997) 103-109
106
Q1 30.5
I
(1)
I
Qt 30.5
I
•
(1)
I
I
I
(2)
(2)
I
I
I
I
I
I
(3)
(3)
L/ i
i
-2
0
km/s
krn/s
Fig. 2. Nascent Doppler profiles for CN(v = 0, N = 30) Q-branch lines detected in the three indicated excitation-detection geometries. The circles represent the experimental data and the solid lines are the global fits to these data and the R-branch data (not shown) probing the same initial state,
to its limiting perpendicular value of - 0 . 5 . The systematic deviations are eliminated when the/3°(22) moment is allowed to float, assuming values near - 0 . 3 5 , as shown in Table 1. Our R-branch data alone are insufficiently sensitive to the v - j correlation to allow a precise determination of this vector correlation. The /302(20) bipolar moment is one half the more familiar velocity anisotropy parameter, /3. In the limit of prompt recoil /3 ranges from 2.0 for a parallel transition to - 1,0 for a perpendicular transition. In Fig. 4 we have plotted the anisotropy parameter as a function of the detected CN(v = 0) rotational state. The values at N = 11 and N = 24 are based on a less complete set of measurements, which still, however, permitted the extraction of reliable values of /302(20) within the error limits indicated. Our average values o f / 3 are close to the value of 1.5
Fig. 3. Optimum fits (solid lines) to the same data shown in Fig. 2, with the center-of-mass /30(22) moment (the v . j correlation) constrained to - 0.5.
reported by Griffiths and E1-Sayed at 304 nm [18], confirming their observation that a parallel transition dominates the excitation at these wavelengths. We
I
o
1.5 P~
I
I
I
I
o
1.0
0 0
0.5 0.0 -0.5
0
I
I
I
I
I
10
20
30
40
50
N Fig. 4. Plot of the anisotropy parameter, 2fl02(20), as a function of the detected CN(v = 0) rotational state.
S. W. North et al. / Chemical Physics Letters 276 (1997) 103-109 Table 2 Alignment, A2, for CN (v = 0, N) fragments from ICN photodissociation at 308 nm
N = 20 N = 30 N=40
Intensity measurement
Bipolar moment analysis
-0.16+0.07 - 0 . 1 5 +0.07 -0.22+0.07
-0.22+__0.09 - 0 . 3 1 ___0.08 -0.30__+0.09
believe the slight trend to higher anisotropy at lower rotational states is significant. We have determined the rotational alignment for selected CN rotational states in v = 0 by two methods. The alignment parameter, A 0, describes the correlation between J and the space-fixed direction z defined by the polarization vector of the dissociation light and ranges from - 0 . 4 to 0.8, corresponding to the respective liming cases where J is perpendicular or parallel to the transition moment. A~0, equivalent to 4 / 5 flo2(02), can be determined from the ratio of experimental signal intensities taken in different excitation-detection geometries. For single photon absorption the integrated intensity of a nascent rotational line shape depends only on the go term in Eq. (4). A comparison of the integrated signals for both Q- and R-branches lines of selected rotational states in collinear geometries 1 and 2 gives the rotational alignment. The final values of A o for N = 20, 30 and 40 represent the average of Q- and R-branch measurements and are presented in Table 2. Our values of A~ are similar to the N-averaged value of - 0 . 1 9 reported by Houston and co-workers at 290 nm [25], and are also consistent with the dominance of a parallel transition leading to photodissociation at 308 nm. The /3o2(02) moment can also in principle be determined from its minor effect on the shapes of high quality normalized Doppler profiles. Floating the /3o2(02) values in the global fit to multiple normalized Doppler profiles yields the values of the alignment presented in Table 1 for comparison with the intensity measurements. In all cases the two independent measurements are within the error limits. The derived values of the other bipolar moments were not significantly affected by fixing /3o2(02) at the value derived from intensity ratios, or allowing it to float. In the absence of parent internal energy there is a
107
single speed associated with each detected CN(v, N) fragment from ICN dissociation at 308 nm. We have determined a value for the D0°(I-CN) of 26,600 + 200 cm l based on recent measurements at 266 nm and 248 nm [19]. This value is in good agreement with the 26,500 + 500 cm -1 advocated by Wittig and co-workers [1]. Interestingly, we find that the 308 nm Doppler profiles require an additional 200 - 300 cm i of available energy on average to achieve optimum fits, suggesting the importance of vibrational hot bands in the dissociation [9]. The disappearance of CN photoproduct signal at an excitation wavelength of 320 nm on jet cooling [26] has also been previously interpreted as evidence that the photodissociation of thermal ICN is dominated by hot bands in the red wing of the absorption band. The vibrationally excited levels of CN were observed at significantly reduced amplitude. The nascent Doppler profiles of these levels also showed a clear positive velocity anisotropy, similar to that observed for v = 0 CN states. The weaker signals made a complete and quantitative lineshape analysis more difficult, however. We estimate the total v = 0:1 population ratio to be 1:0.07 ___0.03, in agreement with measurements of Fisher et al. at 308 nm [9], but far less vibrational excitation than was inferred by Griffiths et al. at 304 nm [18]. An unrecognized experimental artifact played a central role in the data analysis of that work [18]. Some 20% of the observed signal exceeded the maximum available energy by up to a factor of 2, yet was used to estimate the magnitudes and anisotropies of overlapping features. We consider the results of that analysis unreliable.
4. Discussion The strong positive velocity anisotropy and negative rotational alignment observed for all CN states poses some problems for the common assumption that the red wing of the absorption band is dominated by a perpendicular transition to the 3I-I1 state [5,14-16]. One can consider several possible explanations. First, the transition moments to the 31-I1 state could have been overestimated, and the absorption may still be dominated by the 3110+ state at 308 nm, despite poor Franck-Condon factors. The
108
S. W. North et al. / Chemical Physics Letters 276 (1997) 103-109
importance of hot bands adds to the credibility of this explanation. Second, some interference effect arising from coherent excitation of 3II1 and 31-10+ states might be invoked, although the total amplitude of the parallel component cannot be negligible. The P~ and A' components of the 31/l state cannot interfere to give a positive anisotropy, although such an effect is considered to be responsible for the net CN angular momentum orientation observed with circularly polarized light at 248 nm [3,27,28]. Third, the dominance of hot bands offers another possibility. An excitation from the singly excited bending state (with one unit of vibrational angular momentum) of ICN to the 3H 1 state could acquire oscillator strength from the 31/0+ state by vibronic spin-orbit interactions in a A K = +__1 transition. For such a vibronic interaction to be stronger than the pure electronic spin orbit mixing [5] (with the lII1 state) would be unusual, but perhaps worth considering. Temperature dependent studies allow significant changes in the vibrational populations, and could test some of these possibilities. Initial classical calculations on the 31/1 surface [5] provided an explanation for the local maximum in the vibrational excitation of CN at the red edge of the excitation spectrum, although the calculations were performed at total energies characteristic of the peak of the absorption band. If the role of the 31-I1 state is minimized to explain the anisotropy of the photofragments at 304 and 308 nm, a new explanation is required to account for the small excess vibrational excitation between 290 and 310 nm. Our measurements indicate that the vector properties of the small v = 1 population are not significantly different from those of the v = 0 population, in disagreement with the conclusions of the REMPI-TOF experiments at 304 nm [18]. There have been numerous previous discussions of the possibility and implications of photofragment v.j correlations, fl°(22), deviating significantly from - 0 . 5 in the case of linear triatomic dissociation [3,4,29,30]. In the limit of zero initial parent molecule angular momentum, the problem is nearly trivial. Neglecting the angular momentum of the dissociation photon and the iodine atom compared to high angular momentum of a diatomic fragment, angular momentum conservation requires that the orbital angular momentum of the fragment pair, l,
and the rotational angular momentum of the diatom, j, be antiparallel and equal in magnitude. Since l is perpendicular to the center-of-mass relative velocity, v, a sharply perpendicular relation between v and j is required. More generally, angular momentum conservation strictly forbids j to have any component along v in excess of the algebraic s u m Jatom + Jp .... t + 1. This restriction on the molecular helicity can still allow for significant depolarization without violating angular momentum conservation. A phase space theory prediction for the v .j correlation of CN v = 0, N = 30, for example, is - 0 . 1 for a room temperature initial distribution of ICN rotational states [31]. In order for this statistical degree of depolarization to be achieved, however, all allowed helicity states of the recoiling pairs must be accessible, starting from bound states with body-fixed angular momentum projection numbers /2 = 0 or 1 only. The CN v . j correlation thus gives a direct experimental measure of the number of asymptotic helicity states contributing to the photodissociation. The observed v .j correlation moment of - 0 . 3 5 at N = 30 could, for example, result from equal contributions from all total helicity states up to + 16, or from a Gaussian distribution of helicity states characterized by a sigma of 10. Such depolarization is also observed at shorter dissociation wavelengths, but to a lesser extent [19]. The reprojection of the initial body-fixed fragment angular momentum onto the final recoil velocity can be thought of as a Coriolis mixing of helicity states in the exit channel, which depends on the deviations from the axial recoil limit [32,33].
5. Concluding remarks High-resolution transient FM spectroscopy of the CN photofragments from room temperature ICN photodissociation at 308 nm has provided a set of Doppler profiles that characterize the vector properties of the excitation and dissociation dynamics. Close analysis of the wings of the Doppler profiles indicate that vibrational hot bands dominate the excitation. The velocity anisotropy for all CN states detected in v = 0 and v = 1 indicates the dominance of a parallel optical transition. Surprisingly large deviations from limiting v .j correlations are ob-
S.W. North et a l . / Chemical Physics Letters 276 (1997) 103-109
served with high confidence, which we attribute to Coriolis interactions associated with deviations from the axial recoil limit.
Acknowledgements This work was performed at Brookhaven National Laboratory under Contract No. D E - A C 0 2 76CH00016 with the U.S. Department of Energy and supported by its Division of Chemical Sciences, Office of Basic Energy Sciences.
References [1] I. Nadler, D. Mahgerefteh, H. Reisler, C. Wittig, J. Chem. Phys. 82 (1985) 3885. [2] M.A. O'Halloran, H. Joswig, R.N. Zare, J. Chem. Phys. 87 (1987) 303. [3] J.F. Black, J.R. Waldeck, R.N. Zare, J. Chem. Phys. 92 (1990) 3519. [4] J.F. Black, J. Chem. Phys. 98 (1992) 6853. [5] Y. Amatatsu, S. Yabushita, K. Morokuma, J. Chem. Phys. 100 (1994) 4894. [6] J.H. Ling, K.R. Wilson, J. Chem. Phys. 63 (1975) 101. [7] A.P. Baronavski, J.R. McDonald, Chem. Phys. Lett. 45 (1977) 172. [8] W.H. Pitts, A.P. Baronavski, Chem. Phys. Lett. 71 (1980) 395. [9] W.H. Fisher, T. Carrington, S.V. Filseth, C.M. Sadowski, C.H. Dugan, Chem. Phys. 82 (1983) 443. [10] W.H. Fisher, R. Eng, T. Carrington, C.H. Dugan, S.V. Filseth, C.M. Sadowski, Chem. Phys. 89 (1984) 457.
109
[I 1] A.P. Baronavski, Chem. Phys. 66 (1982) 217. [12] S. Yabushita, K. Morokuma, Chem. Phys. Lett. 175 (1990) 518. [13] Y. Wang, C.X.W. Qian, J. Chem. Phys. 100 (1994) 2707. [14] Y. Wang, C.X.W. Qian, Chem. Phys. Lett. 219 (1994) 389. [15] J.M. Bowman, R.C. Mayrhofer, Y. Amatatsu, J. Chem. Phys. 101 (1994) 9469. [16] J. Qian, D.J. Tannor, Y. Amatatsu, K. Morokuma, J. Chem. Phys. 101 (1994)9597. [17] H. Wei, T. Carrington Jr., J. Chem. Phys. 105 (1996) 141. [18] J.A. Griffiths, M.A. E1-Sayed, J. Chem. Phys. 100 (1994) 4910. [19] S.W. North and G.E. Hall, to be published. [20] J.C. Bloch, R.W. Field, G.E. Hall, T.J. Sears, J. Chem. Phys. 101 (1994) 1717. [21] S.W. North, X.S. Zheng, R. Fei, G.E. Hall, J. Chem. Phys. 104 (1996) 2129. [22] S.W. North, G.E. Hall, J. Chem. Phys. 106 (1997) 60. [23] G.E. Hall, M. Wu, J. Phys. Chem. 97 (1993) 10911. [24] R.N. Dixon, J. Chem. Phys. 85 (1986) 1866. [25] G.E. Hall, N. Sivakumar, P.L. Houston, J. Chem. Phys. 84 (1986) 2120. [26] I. Nadler, H. Reisler, C. Wittig, Chem. Phys. Lett. 103 (1984) 451. [27] E. Hasselbrink, J.R. Waldek, R.N. Zare, Chem. Phys. 126 (1988) 191. [28] J. Vigu6, B. Girard, G. Gou6dard, N. Billy, Phys. Rev. Lett. 62 (1989) 1358. [29] J.F. Black, L Chem. Phys. 100 (1994) 5392. [30] H. Joswig, M.A. O'Halloran, R.N. Zare, M.S. Child, Faraday Discuss. Chem. Soc. 82 (1986) 79. [31] S.W. North, G.E. Hall, J. Chem. Phys. 104 (1996) 1864. [32] M.D. Morse, K.F. Freed, J. Chem. Phys. 74 (1981) 4395. [33] S.W. North, G.E. Hall, Ber. Bunsenges. Phys. Chem. 101 (1997) 459.
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 276 (1997) 103-109
Vector correlations in the 308 nm photodissociation of ICN Simon W. North a, Julie Mueller b, Gregory E. Hall a,* a Chemistry Department, Brookhaven National Laboratory, Upton, N Y 11973-5000, USA b Department ofChemisto', Cornell University, Ithaca, N Y 14853, USA Received 6 June 1997; in final form 6 June 1997
Abstract
Nascent Doppler profiles of CN (X 2£ +) fragments from the 308 nm photodissociation of ICN have been measured using high-resolution transient frequency modulated (FM) absorption spectroscopy. The complete set of vector correlations observable with linearly polarized single photon detection has been determined for several rotational states in v = 0. All detected states exhibit strong positive velocity anisotropy indicating that the initial optical excitation is predominantly a parallel transition. The result challenges recent theoretical work which predicts that at wavelengths > 300 nm a perpendicular transition to the 31-Ii state dominates the dissociation. © 1997 Elsevier Science B.V.
1. Introduction
The photodissociation of ICN in the A-continuum has attracted a wide range of increasingly sophisticated experimental and theoretical investigations over the past twenty years, becoming a prototype for multiple-surface dissociation dynamics. The most detailed experimental work has been performed at wavelengths close to the peak of the absorption profile, notably at 266 nm and 248 nm [1-4]. Here, two channels compete, producing iodine atoms in either their ground or spin-orbit excited states: ICN(X ']~+) + hv---~ CN(X 2 ~ + , v , N ) + I(2p3/e),I(epI/2).
(1)
The elucidation of the optical excitations and the multiple-surface dynamics leading to these asymptotic products is a long and unfinished story. Good
* Corresponding author. E-mail: [email protected].
summaries of recent work can be found in references [3] and [5]. At the red edge of the A band, corresponding to wavelengths longer than 300 nm, there has been less detailed experimental work, and the situation is apparently simpler. The energetic threshold for forming CN and excited (2Pl/2) iodine atoms is approximately 295 nm, so the dissociation has only a single asymptotic channel open [6-8]. The CN rotational distribution appears to be single component, which lies intermediate between the high and low rotational components observed at shorter wavelengths [9,10]. Despite the decreased available energy in the red wing of the absorption band, excitation here results in a somewhat higher degree of CN vibrational excitation than observed at 266 nm or 248 nm. At 308 nm Fisher et al. measured the population in v = 0 : l : 2 to be 0.88:0.10:0.02 [9], while Baronavski reported a vibrational ratio of 0.91:0.06:0.03 at 299.4 nm [11]. Recent theoretical work has provided a clearer view of ICN dissociation dynamics. Yabushita and
0009-2614/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 0 0 0 9 - 2 6 1 4 ( 9 7 ) 0 0 7 9 0 - 2
104
S. W. North et al. / Chemical Physics Letters 276 (1997) 103-109
Morokuma have calculated new ab initio excited state potential energy surfaces for ICN and have identified three dipole allowed states l IIl(E), 3I-I0+(Al) and 31-I~(E) that are all thought to contribute to the A band absorption [5,12]. Classical and quantum dynamics calculations using these surfaces or minor variations have successfully reproduced many of the experimental observations [5,13-17]. Theory predicts that the 3[I l state should dominate the absorption at longer wavelengths. Trajectory calculations performed on this lowest energy surface qualitatively reproduce both the vibrational and rotational state distributions observed at the red wing of the excitation spectrum. Griffiths and E1-Sayed have recently measured I(2p3/2) fragments arising from ICN photodissociation at 304 nm by one-dimensional photofragment translational spectroscopy [18]. The spatial anisotropy of the fragments suggested that the dissociation initially involved a predominately parallel transition. They have argued that a significant fraction of the absorption must therefore still be to the 3H0+ state. The purpose of the present paper is to further examine the vector properties of ICN dissociation in the long wavelength tail of the absorption spectrum, extending the work of Black et al. [3,4], to longer wavelengths. Our measurements at 308 nm represent a detailed set of observables which can provide a stringent test of the recent ab initio potential energy surfaces and transition moments. Extensive new FM Doppler measurements at shorter wavelengths have also been performed. The conclusions differ in some important ways from previous reports and will be the subject of a more thorough future report [19].
2. Experiment The general features of the experimental apparatus have been described previously [20-22]. Briefly, the photolysis light was provided by an unfocussed beam from an XeC1 excimer laser operating at 12 Hz. The unpolarized beam was passed through 10 quartz plates at Brewster's angle with respect to the direction of propagation resulting in ~ 95% linear polarization. Adjustment of the photolysis polarization was achieved by rotating the stack of Brewster's
~
probe
idiss
(1)
(2)
Fig. 1. Schematicdiagram illustratingthe three independentexperimental excitation-detectiongeometries. Transient Doppler measurements correspondingto geometries(1) and (2) were taken in the collinearsingle-pass cell. Data correspondingto geometry(3) were taken in the multipass transversecell. plates. The photolysis pulse provided fluences of 10-20 m J / c m 2 to the sample. The nascent (X 2~) CN radical photoproducts were probed in the Av = 2 bands of the A 2 I ] ~-- X 2~ system near 800 nm using the continuous light from a linearly polarized Ti:sapphire ring laser. The transient FM method for Doppler spectroscopy is a zero background, low noise technique for one-photon transient absorption spectroscopy. Full details have been published [21,22]. Neither the single mode ring laser bandwidth nor power broadening makes a significant contribution to the observed Doppler spectra. The use of the A - X system, with Q branches and fully resolved fine structure levels, allows the acquisition of a more complete and less blended set of Doppler lines for a more stable analysis than is possible with the more commonly used B - X system, as noted previously [3]. Three independent excitation-detection geometries were employed in the experiment and are shown schematically in Fig. 1. Measurements in geometries 1 and 2 were performed in a 1.5 m single pass collinear-cell in which the probe and photolysis beams counter propagate. Measurements in geometry 3 were performed in a transverse multipass-cell. In this cell, the frequency modulated probe laser beam made multiple reflections between plane-parallel dielectric coated mirrors aligned parallel to the photolysis laser beam propagation direction [23]. In both arrangements a slow flow (4 sccm) of room temperature ICN (Aldrich) was maintained at a total pressure of 20 mTorr. Inspection of the time-dependence of the Doppler profiles indicated that negligible transla-
S. W. North et aL / Chemical Physics Letters 276 (1997) 103-109
tional or rotational relaxation had occurred in the first 100 ns.
105
convolution over the thermal parent velocity distribution [19]. The result is that the observed laboratory Doppler profiles, D'ob~(w),can be written as a linear combination of three basis functions,
3. Results
D'obs(W) = Several rotational states of CN (v = 0, N) were measured with Q and R branch transitions in geometries 1, 2, and usually 3. The raw transient FM data were transformed to Doppler absorption profiles using techniques described in refs and. The Doppler profiles were analyzed using the bipolar moment formalism of Dixon [24] as applied in the case of linearly polarized one-photon absorption spectroscopy [23]. The Doppler profile for an ensemble with a single speed, v, can be written as
l[go+g2P2(-~,)+g4P4(W)l D(w) = 2--7'
(2)
where w is the component of the laboratory velocity along the probe direction and P2 and P4 are Legendre polynomials. For linearly polarized, single photon detection, the coefficients gi are given by the following expressions,
go = 1 + b I/3o2(02) go =be/3o2(20) + b3/3°(22) + b4/3O2(22)
(3)
g4 = b5/32(42) where b i are the bipolar multipliers which depend on geometrical factors, including the incomplete degree of polarization, and the rotational branch, and/3ff(k 1, k 2) are the renormalized bipolar moments of the correlated velocity and angular momentum distribution. In the presence of internal energy of the parent there is a distribution of speeds, each speed corresponding to a different initial parent internal state. It is straightforward to generalize Eq. (2) to include a thermal average over parent internal states and the
goD'o( W) + g2D'2( w) + g4D4( w). (4)
We have chosen to simultaneously fit the normalized Doppler profiles for each rotational line in three geometries with both branches to determine the full set of bipolar moments. We iteratively adjust the values of the center-of-mass bipolar moments to minimize the X 2 of the entire data set. We find that this method gives the fairest estimate of the errors and is stable with high quality data [ 19]. A sample of the measured Doppler profiles and fits is shown in Fig. 2. The fourth order term D~, usually neglected in Doppler LIF experiments, makes a significant contribution, particularly in geometry 1. We have corrected for the incomplete polarization of the photolysis laser in the data analysis. The results for fitting Q and R lines in multiple geometries are given in Table 1 for the N = 20, 30 and 40 states (F 1 components) of CN in v = 0. We find that the Doppler analysis is most sensitive to the values of/3~(20) and /30(22) as indicated by the error limits cited. Particularly surprising is the substantial deviation of the v .j correlation, /30(22), from its limiting perpendicular value of - 0 . 5 . This would be strictly forbidden by angular momentum conservation for total angular momentum zero, but can be allowed in a thermal sample of 1CN. Such deviations have been reported [3] and then retracted [4] for ICN at 248 rim, recognizing the difficulty of extracting this information from the B - X spectra, which lack Q branch lines. To illustrate the sensitivity of the data to the /3°(22) moment, Fig. 3 shows an optimized fit of all other bipolar moments to the same data set, with the v . j correlation constrained
Table 1 Bipolar moments for CN (t, = 0, N) fragments from ICN photodissociation at 308 nm
N = 20 N = 30 N = 40
/32(02)
/3~(20)
/3o(22)
/3o2(22)
/32(42)
- 0 . 2 8 ::[:0.12 - 0 . 4 0 + 0.10 - 0 . 3 8 + 0.12
0.77 + 0.03 0.70 + 0.03 0.60 + 0.04
- 0 . 2 7 ± 0.06 - 0 . 3 5 + 0.04 - 0 . 3 2 + 0.04
0.24 ± 0.06 0.29 + 0.06 0.23 + 0.06
- 0 . 3 2 _ 0.08 - 0 . 3 7 ± 0.08 - 0 . 3 9 _ 0.08
S. W. North et al. / Chemical Physics Letters 276 (1997) 103-109
106
Q1 30.5
I
(1)
I
Qt 30.5
I
•
(1)
I
I
I
(2)
(2)
I
I
I
I
I
I
(3)
(3)
L/ i
i
-2
0
km/s
krn/s
Fig. 2. Nascent Doppler profiles for CN(v = 0, N = 30) Q-branch lines detected in the three indicated excitation-detection geometries. The circles represent the experimental data and the solid lines are the global fits to these data and the R-branch data (not shown) probing the same initial state,
to its limiting perpendicular value of - 0 . 5 . The systematic deviations are eliminated when the/3°(22) moment is allowed to float, assuming values near - 0 . 3 5 , as shown in Table 1. Our R-branch data alone are insufficiently sensitive to the v - j correlation to allow a precise determination of this vector correlation. The /302(20) bipolar moment is one half the more familiar velocity anisotropy parameter, /3. In the limit of prompt recoil /3 ranges from 2.0 for a parallel transition to - 1,0 for a perpendicular transition. In Fig. 4 we have plotted the anisotropy parameter as a function of the detected CN(v = 0) rotational state. The values at N = 11 and N = 24 are based on a less complete set of measurements, which still, however, permitted the extraction of reliable values of /302(20) within the error limits indicated. Our average values o f / 3 are close to the value of 1.5
Fig. 3. Optimum fits (solid lines) to the same data shown in Fig. 2, with the center-of-mass /30(22) moment (the v . j correlation) constrained to - 0.5.
reported by Griffiths and E1-Sayed at 304 nm [18], confirming their observation that a parallel transition dominates the excitation at these wavelengths. We
I
o
1.5 P~
I
I
I
I
o
1.0
0 0
0.5 0.0 -0.5
0
I
I
I
I
I
10
20
30
40
50
N Fig. 4. Plot of the anisotropy parameter, 2fl02(20), as a function of the detected CN(v = 0) rotational state.
S. W. North et al. / Chemical Physics Letters 276 (1997) 103-109 Table 2 Alignment, A2, for CN (v = 0, N) fragments from ICN photodissociation at 308 nm
N = 20 N = 30 N=40
Intensity measurement
Bipolar moment analysis
-0.16+0.07 - 0 . 1 5 +0.07 -0.22+0.07
-0.22+__0.09 - 0 . 3 1 ___0.08 -0.30__+0.09
believe the slight trend to higher anisotropy at lower rotational states is significant. We have determined the rotational alignment for selected CN rotational states in v = 0 by two methods. The alignment parameter, A 0, describes the correlation between J and the space-fixed direction z defined by the polarization vector of the dissociation light and ranges from - 0 . 4 to 0.8, corresponding to the respective liming cases where J is perpendicular or parallel to the transition moment. A~0, equivalent to 4 / 5 flo2(02), can be determined from the ratio of experimental signal intensities taken in different excitation-detection geometries. For single photon absorption the integrated intensity of a nascent rotational line shape depends only on the go term in Eq. (4). A comparison of the integrated signals for both Q- and R-branches lines of selected rotational states in collinear geometries 1 and 2 gives the rotational alignment. The final values of A o for N = 20, 30 and 40 represent the average of Q- and R-branch measurements and are presented in Table 2. Our values of A~ are similar to the N-averaged value of - 0 . 1 9 reported by Houston and co-workers at 290 nm [25], and are also consistent with the dominance of a parallel transition leading to photodissociation at 308 nm. The /3o2(02) moment can also in principle be determined from its minor effect on the shapes of high quality normalized Doppler profiles. Floating the /3o2(02) values in the global fit to multiple normalized Doppler profiles yields the values of the alignment presented in Table 1 for comparison with the intensity measurements. In all cases the two independent measurements are within the error limits. The derived values of the other bipolar moments were not significantly affected by fixing /3o2(02) at the value derived from intensity ratios, or allowing it to float. In the absence of parent internal energy there is a
107
single speed associated with each detected CN(v, N) fragment from ICN dissociation at 308 nm. We have determined a value for the D0°(I-CN) of 26,600 + 200 cm l based on recent measurements at 266 nm and 248 nm [19]. This value is in good agreement with the 26,500 + 500 cm -1 advocated by Wittig and co-workers [1]. Interestingly, we find that the 308 nm Doppler profiles require an additional 200 - 300 cm i of available energy on average to achieve optimum fits, suggesting the importance of vibrational hot bands in the dissociation [9]. The disappearance of CN photoproduct signal at an excitation wavelength of 320 nm on jet cooling [26] has also been previously interpreted as evidence that the photodissociation of thermal ICN is dominated by hot bands in the red wing of the absorption band. The vibrationally excited levels of CN were observed at significantly reduced amplitude. The nascent Doppler profiles of these levels also showed a clear positive velocity anisotropy, similar to that observed for v = 0 CN states. The weaker signals made a complete and quantitative lineshape analysis more difficult, however. We estimate the total v = 0:1 population ratio to be 1:0.07 ___0.03, in agreement with measurements of Fisher et al. at 308 nm [9], but far less vibrational excitation than was inferred by Griffiths et al. at 304 nm [18]. An unrecognized experimental artifact played a central role in the data analysis of that work [18]. Some 20% of the observed signal exceeded the maximum available energy by up to a factor of 2, yet was used to estimate the magnitudes and anisotropies of overlapping features. We consider the results of that analysis unreliable.
4. Discussion The strong positive velocity anisotropy and negative rotational alignment observed for all CN states poses some problems for the common assumption that the red wing of the absorption band is dominated by a perpendicular transition to the 3I-I1 state [5,14-16]. One can consider several possible explanations. First, the transition moments to the 31-I1 state could have been overestimated, and the absorption may still be dominated by the 3110+ state at 308 nm, despite poor Franck-Condon factors. The
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S. W. North et al. / Chemical Physics Letters 276 (1997) 103-109
importance of hot bands adds to the credibility of this explanation. Second, some interference effect arising from coherent excitation of 3II1 and 31-10+ states might be invoked, although the total amplitude of the parallel component cannot be negligible. The P~ and A' components of the 31/l state cannot interfere to give a positive anisotropy, although such an effect is considered to be responsible for the net CN angular momentum orientation observed with circularly polarized light at 248 nm [3,27,28]. Third, the dominance of hot bands offers another possibility. An excitation from the singly excited bending state (with one unit of vibrational angular momentum) of ICN to the 3H 1 state could acquire oscillator strength from the 31/0+ state by vibronic spin-orbit interactions in a A K = +__1 transition. For such a vibronic interaction to be stronger than the pure electronic spin orbit mixing [5] (with the lII1 state) would be unusual, but perhaps worth considering. Temperature dependent studies allow significant changes in the vibrational populations, and could test some of these possibilities. Initial classical calculations on the 31/1 surface [5] provided an explanation for the local maximum in the vibrational excitation of CN at the red edge of the excitation spectrum, although the calculations were performed at total energies characteristic of the peak of the absorption band. If the role of the 31-I1 state is minimized to explain the anisotropy of the photofragments at 304 and 308 nm, a new explanation is required to account for the small excess vibrational excitation between 290 and 310 nm. Our measurements indicate that the vector properties of the small v = 1 population are not significantly different from those of the v = 0 population, in disagreement with the conclusions of the REMPI-TOF experiments at 304 nm [18]. There have been numerous previous discussions of the possibility and implications of photofragment v.j correlations, fl°(22), deviating significantly from - 0 . 5 in the case of linear triatomic dissociation [3,4,29,30]. In the limit of zero initial parent molecule angular momentum, the problem is nearly trivial. Neglecting the angular momentum of the dissociation photon and the iodine atom compared to high angular momentum of a diatomic fragment, angular momentum conservation requires that the orbital angular momentum of the fragment pair, l,
and the rotational angular momentum of the diatom, j, be antiparallel and equal in magnitude. Since l is perpendicular to the center-of-mass relative velocity, v, a sharply perpendicular relation between v and j is required. More generally, angular momentum conservation strictly forbids j to have any component along v in excess of the algebraic s u m Jatom + Jp .... t + 1. This restriction on the molecular helicity can still allow for significant depolarization without violating angular momentum conservation. A phase space theory prediction for the v .j correlation of CN v = 0, N = 30, for example, is - 0 . 1 for a room temperature initial distribution of ICN rotational states [31]. In order for this statistical degree of depolarization to be achieved, however, all allowed helicity states of the recoiling pairs must be accessible, starting from bound states with body-fixed angular momentum projection numbers /2 = 0 or 1 only. The CN v . j correlation thus gives a direct experimental measure of the number of asymptotic helicity states contributing to the photodissociation. The observed v .j correlation moment of - 0 . 3 5 at N = 30 could, for example, result from equal contributions from all total helicity states up to + 16, or from a Gaussian distribution of helicity states characterized by a sigma of 10. Such depolarization is also observed at shorter dissociation wavelengths, but to a lesser extent [19]. The reprojection of the initial body-fixed fragment angular momentum onto the final recoil velocity can be thought of as a Coriolis mixing of helicity states in the exit channel, which depends on the deviations from the axial recoil limit [32,33].
5. Concluding remarks High-resolution transient FM spectroscopy of the CN photofragments from room temperature ICN photodissociation at 308 nm has provided a set of Doppler profiles that characterize the vector properties of the excitation and dissociation dynamics. Close analysis of the wings of the Doppler profiles indicate that vibrational hot bands dominate the excitation. The velocity anisotropy for all CN states detected in v = 0 and v = 1 indicates the dominance of a parallel optical transition. Surprisingly large deviations from limiting v .j correlations are ob-
S.W. North et a l . / Chemical Physics Letters 276 (1997) 103-109
served with high confidence, which we attribute to Coriolis interactions associated with deviations from the axial recoil limit.
Acknowledgements This work was performed at Brookhaven National Laboratory under Contract No. D E - A C 0 2 76CH00016 with the U.S. Department of Energy and supported by its Division of Chemical Sciences, Office of Basic Energy Sciences.
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