J . Phys. Chem. 1990, 94. 6608-661 5
6608
Photodissociation Dynamics of C2N2in the Threshold Region for Dissociation Elizabeth A. J. Wannenmacher, Hua Lin,+ and William M. Jackson* Department of Chemistry, University of California, Dauis, California 9561 6 (Receioed: November 27, 1989; In Final Form: March 20, 19901
The photodissociation dynamics of cyanogen is studied in the spectral region from 48 657 to 47 023 cm-I. The peaks in the C2N2absorption spectrum in the range of the dissociation threshold are assigned as vibrational bands arising predominantly from the A I E L electronic level. Photofragment excitation (PHOFEX) spectra are taken in the region of two absorption peaks: at 48 600 and at 47 740 cm-'. The complex structure of the PHOFEX spectra is evidence that the BlA, Xisg+ electronic transition is occurring, and furthermore, it appears that a mutual interaction between the Ai?;,-( 1 1010) and BIAu(OOO1O)states is occurring, and/or that the BIA, potential function is split into two by a Renner-Teller interaction. Rotational state populations of CN(X2Z+jare measured following the photodissociation of cyanogen at vibrational state selected levels in the region from 48 600 to 47 023 cm-I. Phase space theory (PST) is used with a dissociation energy of 47 100 f 300 cm-' and an impact parameter of 1.8 8, to fit the rotational energy distributions, which are derived from the measured populations. The data show that the only dynamical constraints on the product state rotational energy distributions are conservation of angular momentum and energy. Measurements of the rotational alignment show that the correlation of the fragment angular momentum with the parent transition moment is lost. The PST models, the sharp structure of the PHOFEX spectra, and the loss of alignment suggest that C2N2dissociates through a loose transition state with a small exit channel barrier and the time for dissociation is much longer than the cyanogen rotational period. Equal partitioning of the rotational energy into the fragments, which are dissociated from different electronic levels, indicates that excited-state cyanogen undergoes internal conversion to the vibrational continuum of the ground electronic state.
-
Introduction Experimentall and theoretical* investigations of the photodissociation of small molecules are beneficial for understanding the microscopic dynamics of chemical reactions. The quality of the information obtained from photodissociation investigations was greatly advanced when the laser-induced fluorescence (LIF) technique was presented to probe the photo fragment^.^ Photodissociation investigations using LIF enables reactions of small molecules to be studied where the final quantum state distribution of the products can be fully resolved. The partitioning of available energy, Ea",among the various degrees of freedom of the fragment and the use of the appropriate model to fit the product distributions may provide general information about the internal state distribution of the parent molecule, the lifetime of the excited state, and a determination of whether the dissociation is predissociative or direct. Electronic predissociation occurs after one or more rotational periods during which time the molecule moves from the initially excited electronic state to another electronic state. Direct dissociation is generally faster than one rotation period. The correlation of the product angular momentum vector with respect to the transition dipole moment is likely to be reduced for a predissociative molecule. Studies of the alignment, by means of polarization measurements, can also furnish details concerning the lifetime, geometry, and symmetry of the excited state. Understanding the electronic spectrum of cyanogen, which extends from 300 to 100 nm, is also important to the understanding of the photodissociation dynamics. There are four identified electronic transitions in the near-ultraviolet region. The longest wavelength system, at 300 nm, was assigned by Callomon and Davey4 from the observed rotational structure as a 32u+ IZgf transition. At 250.8 nm, there is a weak system which has about the same intensity as the 300-nm system and its rotational structure suggests this is a 3A, IZg+ t r a n s i t i ~ n . ~ A new electronic transition begins at about 220 nm. This system is much stronger than the last two systems. The system's complicated vibrational structure indicates that the transition is symmetry forbidden but made allowed by the excitation of the v4(x8j vibrational mode. Based on the rotational structure of the 4y transition, the electronic transition ]E,- I?;*+ was assigned by Cartwright et a1.6 and Bell et aL7 High-resolution photographic spectra of a number of bands near the origin of this system at 220.3 nm were reported by Fish et al.* At 207 nm a new elec-
tronic transition appears. The symmetry of the upper electronic state was assigned as a 'A, state.' It is similar in intensity and vibrational structure to the 200-nm system and the vibrational structure shows that the symmetry-forbidden transition is also made allowed by the u4(xg)vibrational mode. The vibrational structure of the electronic transitions at wavelengths longer than 200 nm involves progressions with approximately four members in vi and fewer members in v2.' At energies greater than 200 nm three systems have been o b ~ e r v e d . ~ Previous studies of the photodissociation of cyanogen a t 193 nm have shown that the CN radicals are produced in the 0'' = 0 and 1 The rotational and translational distributions that were obtained could be fitted by phase space theory.I0 In the present paper, photofragment excitation spectra, product rotational distributions, and the alignment of the product radicals from the photodissociation of cyanogen are studied. Cyanogen is chosen as the molecule under investigation because it is a typical small linear tetraatomic molecule, and it photodissociates to produce two C N fragments which can be easily probed with the LIF technique. The goal of this work is to photodissociate C2N2 in the wavelength region of the A'ZL and BIAuelectronic states to determine how the photodissociation dynamics change with different excited-state symmetries and with variations in the available energy. The UV absorption spectra in this region are structured, but the lines are broadened by prediss~ciation.~ The
'Present address: Second Department of Physics, Fudan University, Shanghai 200433, People's Republic of China.
5552. ( ! I ) Halpern. J B.; Jackson. W. M . J . Phys. Chem. 1982. 86, 973.
-
+-
-
( I ) (a) Leone, S. R. Ada. Chem. Phys. 1982, 50, 255. (b) Simons, J . P. J . Phys. Chem. 1984, 88, 1287. ( c ) Jackson, W. M.; Okabe. H. Advances in Photochemisiry; Wiley: New York, 1986; Vol. 13. (2) (a) Baht-Kurti, G. G.; Shapiro, R. Ada. Chem. Phys. 1985, 60, 403. (b) Pechukas, P.; Light, J. C. J . Chem. Phys. 1965,42,3281. ( c ) Pechukas, P.; Light, J. C.; Rankin, C. J . Chem. Phys. 1966, 44, 794. (d) Light, J . C. Faraday Discuss. Chem. Soc. 1967, 44, 14. (3) Jackson, W. M . J . Chem. Phys. 1973, 59, 960. (4) Callomon, J . H.; Davey, A . B. Proc. Int. Conf Spectrosc. Bombay 1969, 173. (5) Cartwright, G. J.; O'Hare, D. 0.;Walsh, A. D.; Warsop, P. A . J . Mol. Spectrosc. 1971, 39, 393. (6) Cartwright, G. J.; Walsh, A. D.; Warsop, P. A. Proceedings of the 8th European Congress on Molecular Spectroscopy, Copenhagen; Butterworths:
London, 1965; Abstr. 334. (7) Bell. S.;Cartwright, G . J.; Fish, G. B.; O'Hare, D. 0.;Ritchie. R. K.; Walsh, A . D.; Warsop, P. A. J . Mol. Spectrosc. 1969, 30, 162. (8) Fish, G. B.; Cartwright, G. J.; Walsh. A. D.; Warsop, P. A. J . Mol. Specrrosc. 1912, 41, 20.
(9) Connors, R. E.; Roebber, J. L.; Weiss, K. J . Chem Phys. 1974, 60, 5011. ( I O ) Eres, D.; Gurnick. M.; McDonald, J. D. J . Chem. Phys. 1984, 81,
0022-3654/90/2094-6608$02.50/0 C I990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. 17, 1990 6609
Photodissociation Dynamics of C2N2
-
correlation diagramI2 indicates that the production of two C N (X2Z+)can proceed via a vibronically allowed A'&XIZg+ or BI A,,- X I Z g +transition followed by internal conversion to high vibrational levels of the electronic ground state. In the present experiment, selected vibronic bands are excited and the product rotational distributions are compared with phase space models.
-
Experimental Section The experimental apparatus was described in a previous article and will only be summarized here.I3 Briefly, the linearly polarized photolysis source in the region of 48 657-47 023 cm-l is obtained by frequency doubling the output of a Lambda Physik EMG 201 MSC XeCl excimer pumped Lambda Physik FL2002 dye laser with a BBO crystal. There is no autotracking unit for adjusting the phase matching angle of the doubling crystal, so the optimization must be performed manually. A Pellin-Broca prism is used to separate the frequency-doubled beam from the fundamental beam. At 210 nm the fwhm bandwidth of the dye laser is approximately I .O cm-l. The C N photofragment is probed by using the LIF technique through the Au = 0 vibrational sequence of the B2Z+ X2Z+ electronic transition. The bandhead of the (0,O) transition is located at 388.4 nm. The probe laser is a Lambda Physik EMG 101 MSC XeCl excimer pumped Lumonics Hyperdye 300 dye laser. The fwhm bandwidth of the probe laser is estimated to be 0.13 f 0.01 cm-l by deconvoluting 300 K line profiles of CN. The output of the probe beam is >96% vertically polarized. When polarization experiments are performed, the beam passes through a Glans-Taylor calcite polarizer to remove residual unpolarized components and then goes into a birefringent photoelastic modulator (Hinds PEM-80), which is set at an angle of 45' with respect to the polarization direction. This modulator alternately rotates the polarization of the light by f90' on a shot to shot basis. The probe laser light enters the chamber in a counterpropagating direction with respect to the photolysis beam. The fluorescence excited by the probe laser is collected at 90' with respect to both laser beams with a set of collection lenses and is passed through a 380-nm Schott color glass filter onto a PMT (EMI-9789QB). The LIF signal is processed by a Stanford Research SR250 boxcar integrator, digitized by a Laser Interface LlllOO and LI1200 interface system, and then stored in an IBM-XT compatible computer. All of the timing pulses for the lasers and gated integrator are also supplied by this control system. The intensities of the photolysis and probe lasers are monitored on a shot to shot basis and used to normalize the LIF signals. The intensities of the photolysis beam (0.5-2 mJ/cm2) and probe beam ( < I pJ/cm2) are carefully set so that during the experiment the LIF signal is in the linear response region. The equation of Marinelli et aI.l4 suggests that the C N transition saturation limit by the probe laser is approximately 1.25 pJ, and several polarizers are introduced to appropriately reduce the intensity of the probe laser beam. No B-X fluorescence is observed when only the photolysis laser is fired. The highest available energy occurs at a photolysis frequency of 48 520 cm-l, where the calculated effective parent vibrational energy of C2N2is approximately 800 cm-'. (The parent vibrational energy at a photolysis frequency of 48 520 cm-' is calculated by using the phase space theory. It is described later in this paper.) The total available energy, which is 2220 cm-I, is enough to produce translational, rotational, and vibrational, but not electronic excitation of the C N fragments. Thus, this observation suggests that two-photon absorption is negligible. C 2 N 2(Matheson, 99.5%) is purified by distillation from an acetone/dry ice trap to a liquid nitrogen trap. All of the nascent LIF measurements are made in a flow system with a parent pressure of 50 mTorr and a delay of 50 ns between the photolysis
-
(12) Dateo, C. E.;Dupis, M.; Lester, W. A., Jr. J . Chem. Phys. 1985.83,
265. (13) Lin, H.; Johnston, E. A.; Jackson, W. M. Chem. Phys. Leu. 1988, 152, 411. (14) Marinelli, W. J.: Sivakumar, N.; Houston, P. L. J . Phys. Chem. 1984,
88,6685.
1,"2,14,1
1,"4,l
1,"41°
2.0
11
i
50,000 49,000 48,000 47,000 46.000 45,000 44,000
Wavenumber in air (cm- l )
Figure 1. Absorption spectrum of C2N2in the region of the dissociation threshold.
-
TABLE I: Possible Vibrational Transition Assignments for C2N2 Absorption through the A%; X'E,' Electronic Transitiona expt, cm-I
44 676 44 893 45 167 45421 45 667 45 935 46 174 46 339 46 588 46 737 47 032 47 435 47 744 47 830 47 933 48 170 48 345 48 444 48 622 48 792 49 1 I9 49 495 49 789 49 938
assignment
+2 +7 +3
+IO +I3 -1 20
-4 +8 +8 -4 -73 +55
-5 -28 +I3 +59 -87 -38 +33 -6 -104 +54 -9 -4
1;2:4:
-28
l;2;4:
-70
1;2:4: la214:
+79 +0.6
la2y4:
+IO0
"The assignments for energies less than 46737 cm-' are from Fish et aL8 Most of the assignments at energies equal to and greater than 46737 cm-' are based on an extension through the ul progression of Fish et al.'s assignments. An averaged valve of 2059 cm-I is chosen for u ~ ' . ' ~The number to the right of an assignment is the measured or extended absorption frequency from Fish minus the measured absorption frequency from H a 1 ~ e r n . l ~The bandhead frequencies are from the absorption spectrum of H a 1 ~ e r n . l ~Where there is more than one possible assignment for a single peak, the most probable assignment is listed in the first column.
laser and the probe laser. The PHOFEX spectra are taken at pressures of 50 mTorr of C2N2and 10 Torr of Ar and with a laser delay of 1 p s . Results and Discussion The Photofragment Excitation Spectra of CN. To understand the photodissociation dynamics of C2N2,it is important to understand the spectroscopy of the molecule in the photolysis region. The absorption spectrum of C2N2has recently been measured near the dissociation limit, 47 100 cm-l, by Halpern15 and is shown in Figure 1. The spectrum is used to extend the vibrational assignments of Fish et al.* to shorter wavelengths by using a u I progression in the region near the band origin of the A'Z; X'Zg+ transition. An averaged value of 2059 cm-I l6 is chosen
-
(15)Halpern, J. B. Private communication.
6610
Wannenmacher et al.
The Journal ofPhysica1 Chemistry, Vol. 94. No. 17, 1990
TABLE 11: The Spectroscopic Constants of the
mode
L>I
symmetry
ZX+
freq ( X ' Z x + ) freq (A'Z,J freq ( B ' A " )
X'E,',
c2
z*+
A'Z;,
and BIAu Electronic States of C2N2'
G3
L'4
E"+
"s
05
n,
sym C N str
CC str
asym CN str
trans-bend
cis-bend
2329.928 205916 236612
845.4x 90716 973'2
2157.84x 1 2783s
502.83* 275 .808 46412
233.158 2224 261"
141012
T, 0 45399.858 48309'
~m
~OOolO
0.157128 0.15383x 0.146912
0.15749x 0.15444x 0.146912
'The superscript to the right of an entry is the reference for that value. All energies are in units of cm-I. All of the values from ref 12 are determined from ab initio MCHF calculations with a large basis set. The rotational constant of the B'A, level is determined from the bond lengths from ref I ? . The value from ref 35 was determined from ab initio MCHF calculations with a small basis set. The error is approximately f250 cm-'.
for 11,' and the anharmonic terms are ignored. These assignments are shown in Table I . At some dissociation wavelengths there are several possible assignments. Photofragment excitation (PHOFEX) spectra or yield curves, the rotational band profile analysis, and the results from the phase space fitting of the product yield are all used in the analysis to determine which alternative(s) is (are) most probable. The photofragment excitation spectrum is obtained by scanning the photolysis laser wavelength while the wavelength of the LIF probe laser is fixed a t the R,(7) line of the C N B2Z+ X2Z+transition. This line corresponds to the maximum J level of a 300 K Boltzmann distribution. In these particular experiments the system pressure and laser delay are such that C N is rotationally relaxed. Recently Halperni5has shown that the lifetime of excited C 2 N ,produced in the 47 740-cm-' region is long enough so that collisional dissociation can occur. The shape of the PHOFEX spectrum obtained in this region could be affected by this process; however, since the lifetime decreases with increasing frequency it is unlikely that the PHOFEX spectra taken at higher frequencies are altered by such a process. During the experiment, the photolysis laser is manually tuned to fixed frequencies in the region of 48 600 and 47 740 cm-I to X I 2 2 12:4:; and AiZ,collect PHOFEX spectra of the AiZ[ X'Zg+ 14;; bands, respectively. The PHOFEX spectra are shown in the lower portion of Figure 2, a and b. Woo and Badger16 investigated the absorption spectrum of cyanogen in the ultraviolet region. I n the region of 48 600 cm-', there is good agreement between the absorption spectrum of Woo et al. and the PHOFEX spectrum . The C2N2absorption spectra of the 1;2:4; and IA4; vibrational bands of the A'Z,X I 2 electronic transition and of the 4; and 47 vibrational bands of t i e B'A, X'Zgf electronic transition are simulated in an effort to understand the structure and width of the PHOFEX spectra. A 300 K Boltzmann distribution for C2N2is assumed and the line intensities are corrected for relative line strengths, but not for the relative Franck-Condon factors as these values are not known. An alternation of the intensities of the J,,,, and Jdd is also included because of the nuclear spin of the identical nitrogen nuclei. Each rotational line is fitted to a Gaussian function with the fwhm of the function equal to the bandwidth of the photolysis laser. For each spectra, the intensities whose corresponding energies lie in a range of 1 .O cm-' are summed together. All of the constants used to calculate spectra XIZ: electronic transition are taken corresponding to the AiZ; from experimental data83I6and should therefore be accurate. The X12,+ transition are obtained from the constants for the B'A, theoretical calculations of Dateo et al.I2 and the measurements of Bell et al.' and Fish et The error in Dateo's et al. vibrational constants is approximately 5% and in TJB'A,) it is >IO0 cm-'. The data used are shown in Table 11. Hereafter, the A'Z,-+ XIZg+,1;2;4:, 41,!; and BiAu XiZ,+, 4;, 4: transitions will be referred to as 1 ;2;4;, 1 ;4;, 4;, and 47 transitions, respectively. The and 4; transitions are shown as simulated spectra for the 1:2:4; the upper spectra of Figure 2a. The simulated spectra for the 1b4: and 4: transitions are displayed as the upper spectra of Figure 2b. A comparison of the PHOFEX spectrum at 48 600 cm-' with the 1;2;4; simulated spectrum suggests that the peaks in the
BIAu
-XIEg+, 101201401 P/Q
AIEu-
i
- X I E g + 401 ,
~
-R
P/Q
- R
+ -
-
c
c
,
~
:
48400
i
"
~
l
:
:
:
:
~
48500
'
'
:
