J. Phys. Chem. 1994,98, 1525-1531
1525
Photofragment Translational Spectroscopy of n-Butyl Iodide at 277 and 304 nm: Polarization Dependence and Energy Partitioning Wee Kyung Kang, Kwang Woo Jung,' and Kyung-Hoon J u g ' Center for Molecular Science and Department of Chemistry, Korea Advanced Institute of Science and Technology, Taeduck Science Town, Taejon 305-701, Korea Hyun Jin Hwang Department of Chemistry, Kyung Hee University, Seoul 130-701, Korea Received: September 14, 1993; In Final Form: November 16, 1993" Photodissociation of n-CdHgI at excitation wavelengths of 277 and 304 nm has been studied utilizing stateselective photofragment translational spectroscopy. The quantum yields of I*(2P1/2) at these wavelengths are determined to be 0.61 and 0.14, respectively. The anisotropy parameters observed at 277 nm are $(I) = 1.6 f 0.1 for the ground-state I(2P3/2) and $(I*) = 0.9 f 0.1 for the excited-state I*(2P1/2) which substantially differs from the limiting value. The results are interpreted in terms of dual path formation of iodine atoms from two different excited 3Q0 and 1Q1 states, i.e., a direct and an indirect dissociation via curve crossing between these states. The @I) (=1.9 f 0.1) at 304 nm, close to its limiting value, shows that the primary processes for I and I* formation channels proceed dominantly via a parallel transition to the 3Q0 state in the longwavelength region of the A band absorption profile. The translational energy distributions of recoil fragments show that a large fraction of the available energy goes into the internal excitation of the photofragment ( ( E i n t ) / Ea", E 0.7) for both dissociation channels. The quantitative analysis is presented for the state-dependent branching ratio and curve crossing probability combined with a quasidiatomic Landau-Zener description.
I. Introduction Recent investigations on the photodissociation dynamics of alkyl iodides in the lowest lying A absorption band14 have yielded detailed information about the energy partitioning in photofragments, the branching ratios for I(ZP112) to I(2P3i2)formation, and the product angular distributions with respect to the electric field vector. Alkyl iodides have received substantial attention for systematic study of photodissociation since very rapid dissociations along therepulsive~tates~vsoffer a chance to monitor the effects of dynamics due to molecular and electronic structures. Excitation in the A band absorption profile, centered near 260 nm, leads to a prompt breaking of the C-I bond in alkyl iodides. The repulsive nature of theexcited states produces an electronically ground-state alkyl radical and either a spin-orbit excited-state I(2Pl/2) atom (denoted I*) or a ground-state I(2P3p) atom (denoted I).1+9TheA band absorption spectra among alkyl iodides arevery similar,1° implying that the electronic transitionsinvolved are nearly the same. These similar absorption behaviors are attributed to the excitation of an electron from a lone pair nonbonding orbital in the iodine atom to the u* antibonding orbital localized in the C-I bond. The contributions to the molecular excitation of alkyl iodides are only from 3Q1,3Q0, and 'Q1 states in Mulliken's notation11J2 out of five available states from the u* configuration. The 3Q0 state correlates asymptoticallywith formation of an I* atom, and the transition momentum from the ground state lies parallel to the C-I bond axis. The 3Q1and 'Q1 states correlatewith formation of I atoms through perpendicular transitions. The magnetic circular dichroism experiments13J4 on methyl, ethyl, and tertbutyl iodides have been utilized to determine the relative contributions of these transitions to the A band absorption. The study reveals that 3Q0 absorption accounts for 70-80% of the oscillator strength at near A band maximumand thecontributions from 3Ql and 'Q1 states are also appreciable at the band edges. Author to whom correspondence should be addressed. t Present address: Department of Chemistry, Wonkwang University, Iri 570-749, Korea. Abstract published in Aduance ACS Abstracts, January 1 , 1994.
0022-3654/94/2098- 1525$04.50/0
Most photofragment spectroscopic studies to date have concentrated only on the angular and energy distributions of the primary photofragments of small alkyl halides. In methyl iodide for example, the overwhelming transition (>95%) has been due to the 3Q0 state,'J5-l7 in which the ground-state alkyl radicals and I* atoms are produced. In higher homologs a large number of I atoms are produced by a nonadiabatic curve crossing during dissociation.18-20An interesting aspect of alkyl iodide photolysis in the A band is that the internal energy partitionings in alkyl radicals have the correlations with their structural properties. It is also found that the available energy distributes more effectively into the internal modes of alkyl fragments as the alkyl group becomes bulkier, especially about the a-carbon.'* Energy partitionings in alkyl iodide photolysis have been reported only on methyl to propyl iodides due to the limited resolution of a molecular beam/quadrupole mass spectrometer. n-Butyl iodide thus has been chosen as a prototype compound to test the effects of radical structure, excited states, energy partitioning, and curve crossing probability on the dissociation dynamics. In this work, we investigate the photodissociation of n-butyl iodide at 277 and 304 nm to determine the angular and kinetic energy distributions of photofragments. The wavelength dependent quantum yields for I and I* formation channels are also examined by a state-selective photofragment translational spectroscopy with a polarized laser and supersonic expansion/pulsedfield time-of-flight (TOF) mass spectrometer. We have chosen the long-wavelength region of the A band in order to access the photon energy effect and the contributions of excited states to the absorption profile and to determine the role of these states on the I/I* branching ratios. A comparison between the experimental observation and theoretical prediction, obtained from model calculation, is also made on the energy partitioning of available energy into the photofragments. 11. Experimental Section
(A) Apparatus. The experimental setting, similar in design to that described previously,21 consists of a supersonic molecular beam source and a TOF mass spectrometer. A schematic view 0 1994 American Chemical Society
Kang et al.
1526 The Journal of Physical Chemistry, Vol. 98, No. 6,1994
-
--
-
I I
TMP
Photodiode
Generator
n
-
I
1
1 '
o
1
1
Pulsed Nozzle Driver
1 I
Pulse Generator
I
-1
Laser Controller
~
Figure 1. Schematic diagram of the experimental apparatus.
of the experimental arrangement is presented in Figure 1. The supersonic molecular beam is operated with a 10-Hz repetition rate and at 400-Torr stagnation pressure. The source chamber with a 4-in. diffusion pump houses a pulsed molecular beam valve providing a gas pulse of fwhm ca. 100 ps and a 0.8-mm-diameter conical skimmer located 5 mm from the nozzle. The skimmed beam passes through the main chamber equipped with a liquid nitrogen trapped 8-in. diffusion pump and is further collimated by a slit before entering the reaction zone, reducing the final beam angular width to ca. 2O. The molecular beam in the field free ionization region of TOF mass spectrometer is crossed at right angles by pulsed UV laser light. The interaction region is located 10.5 cm from the nozzle tip. The mass spectrometer with a vertical flight tube is differentially pumped by a turbomolecular pump, 200 L/s. The source, main, and detection chambers are kept always below 1 X 10-4, 1 X lod, and 3 X l t 7 Torr, respectively. The gas mixture for the molecular beam was prepared by passing Ar or He over liquid n-butyl iodide (ca. 10-Torr vapor pressure at room temperature) to make a total stagnation pressure of 400 Torr. n-Butyl iodide (Aldrich Co.) was purified by several freeze-thaw cycles. The partial pressure of n-butyl iodide was controlled by heating the sample reservoir to an appropriate temperature. The gas inlet tube was kept at a higher temperature than the sampling chamber to prevent condensation. (B) Laser Light Source. A nanosecond Nd:YAG pumped dye laser system (Lumonics HY-750/HD-500) is used as a light source for this study. The wavelength regions of 277 and 304 nm are obtained by frequency-doubling the outputs of rhodamine 590 and 640 (Exciton Co.) dye lasers with a KDP crystal. The polarized laser beam, aligned using a half-wave retardation plate, is focused on the beam intersection zone of the TOF mass spectrometer using a UV-grade lens, focal length = 0.3 m, through a quartz window. The state-selective ionizations of photodissociated iodine atoms are followed via resonance-enhanced multiphoton ionization (REMPI) within the same laser pulse. In the 277-nm region, two wavelengths, 277.38 and 277.87 nm, are used to ionize iodine atoms selectively in the spin-orbit excited I*(5Ps 2P112) and ground I(5Ps 2P3/2) states via two-photon resonance to 6P 2Py/2and 6P D '! states,22 respectively, and one-photon ionization process. In the 304-nm region, the I* and I atoms are also selectively ionized via 6 P 4D7,, and 6p 2D&, states using 304.02 and 304.67 nm. The space-charge effect between ionized photofragments is minimized by adjusting the laser power to a maximum of 0.2 mJ/pulse. (C) Pulsed-Field Time-of-Flight Mass Spectrometry. The ionization region of the single-stage TOF mass spectrometer,
similar to that of Hwang et al.,23,24 consists of two plates, 1 cm apart, grounded during the laser pulse ionization of photofragments. The fragment ions are allowed to spread out in space with the recoil velocities for time delay 7d. After a 0.5-1.0-ps delay from each laser pulse, the ions are accelerated toward the detector applying positivevoltage pulse (Avtech AVRH-2) to the repelling plate. The ions, separated according to their masses, arecollected by the microchannel plate detector located at the end of a 57-cm drift tube. The discrimination plate, with a 6.0-mm-diameter center pinhole, is placed in front of the detector to reduce the detection solid a11gle.2~The arrival time of an ion to the detector is then a function of its initial location prior to the application of the acceleration field. Consequently, the time distribution of the ion packet corresponds to the initial velocity distribution of neutral fragment. Thedelay timeischosenso that anappropriate separation is achieved without losing ion signal. Both I+ components, one traveling toward and another away from the detector, are accelerated into the field-free region applying a 950-V pulse (rise time of 60 ns) for 2 ps. The TOF distributions are measured a t two laser polarization angles, a = Oo and 90°, with respect to the detection axis. The ion signal is amplified (X50) and measured with a time resolution of 10 ns using a transient digitizer and signal averager (LeCroy, Model 9400A). The TOF spectra at 10-Hz typical operation are accumulatedfor 1000pulses. For the specific REMPI wavelength for each iodine atom (I or I* atom), the ion signal is obtained with a wavelength scan of the dye laser. The ion signal is averaged by a gated integrator/boxcar averager (Stanford Research Systems), where the boxcar gate is positioned a t the arrival time of the iodine ions. The dye laser power is monitored during wavelength scans and used to normalize the measured signals. Instrumental parameters, necessary to obtain the photofragment recoil velocity distribution from the measured TOF distributions, are determined from the photoly~is23-*~ of I2 as a reference molecule. III. Results and Analysis
(A) Branching Ratios and Quantum Yields. Figure 2 shows the I+ signal from (2 + 1) REMPI of iodine atoms originated from the laser photolyses of n-C4H9I at 277 and 304 nm regions and a t constant laser power. The n-C4H91shows a well-resolved resonant MPI spectrum in the two excitation wavelength regions, leading to readily observable quantities of I+. Since the u* n absorption spectrumlo of n-C4H91 is continuous over the wavelength range of this study, any sharp enhancement of the I+ signal at discrete laser wavelengths is evidence for the presence
-
The Journal of Physical Chemistry, Vol. 98, No. 6, 1994 1527
Photofragment Translation Spectroscopy of n-C&I
I 304.67
277.50
278.50
305.00
304.00
I'
20.0
WAVELENGTH ( n m ) Figure 2. REMPI spectrum of iodine atoms as a function of excitation laser wavelength for a supersonic molecular beam of n-C&I. Each peak in the spectrum is assigned to the REMPI resonance of iodine atoms originating from their 2P1/2and *P3/2 states.
TABLE 1: Branching Ratios and Fractional Yields from Photodissociation of RCCH~I quantum yield
bx(nm) 277 304
S(I*)/S(I) 16.5 0.202
k 0.094
N(I*)/N(I)
*(I*)
1.55 0.16
0.61 0.14
0.800
*(I) 0.39
0.86
of a neutral I atom. Under our experimental conditions, no ions other than I+ are formed. The iodine mass signal is not observed when the wavelength is slightly off-resonance, indicating no iodine ions are formed by MPI of the parent molecule at these wavelengths. The observed I+ signals in Figure 2 are assigned unambiguously at (2 1) REMPI of iodine atoms originated from their 2P1p and 2P3/2 states.22 These atoms are formed from one-photon dissociation of neutral n-C&I in the red wing of its A band and ionized subsequently by further absorption of photons in the same wavelength region.25 Thus we employed 277.38 and 304.02 nm (specific for I* atom) and 277.87 and 304.67 nm (specific for I atom) as suitable wavelengths for detection of iodine photofragments, corresponding to a high two-photon resonant cross section leading to ionization within the narrow spectral ranges of two photodissociation wavelengths. Themeasuredionsignalratioof I* toIat eachphotodissociation wavelength is proportional to the branching ratio between I* and I atoms by
+
where S refers to the measured intensity, N is the number of iodine atoms resulting from photodissociation, and k is the proportionality constant obtainable from the system calibration using 12 as a standard molecule. The C state of I2 shows a weak and broad absorption continuum in the 230-330-nm region with a maximum near 270 nm.26,27 The studies28.29 on the photodissociation dynamics of 12 have reported that the C X transition is composed of mainly the 1, state which dissociates into I + I* and a very weak (ca. 0.2% contribution) 0, state dissociating into I I. The k values obtained in the present study are 0.094 and 0,800 at 277 and 304 nm, respectively. The branching ratios of n-CdH91 at 277 and 304 nm are calculated using eq 1 and listed in Table 1. From the branching ratios the relative quantum yields @* and ch are determined by the relations +-
+
ch* = N(I*)/[N(I*)
+ N(I)]
and ch = 1 -a*
(2)
From the branching ratios and quantum yields, the I* dissociation
20.5
21.0
21.f
TOF, p s e c Figure 3. Time-of-flight distribution of resonantly-ionized (a) I*(zP1/2)
and (b) I(zP3/2)atoms produced from photodissociation of n-CdH9I at 277.38 and 277.87 nm, respectively, with different laser polarization angles a = Oo (solid curves) and 90° (dottedcurves). The intensity distributions of the two dissociation channels are normalized with respect to a = Oo in order to show more clearly the polarization dependence. The early and late components of the arrival time distribution correspond to the downward and upward velocity components of the iodine recoil fragments. The delay time of the repelling puke was set a t 7.j = 0.5 fis.
channel was found to increase with photon energy, Le., the I*/I ratio rises from 0.16 to 304 nm to 1.55 at 277 nm and shows the same trend as CH31 photodi~sociation.~~ The wavelengthdependent selectivities of the I*/I ratio and quantum yield are due to the absorption band with a t least two overlapping transitions. (B) Photodissociation in the 277-nm Region. For the centerof-mass (cm) translational energy and the cm angular distribution of photofragments from n-CdHgI, the TOFdistributions of iodine ions are measured at 277.38 and 277.87 nm and a t polarization angles, a = 0' and 90°, with respect to the detection axis. It is also necessary to eliminate the interferences from clusters and MPI processes. The state-selected photofragments from clusters exhibit broader velocity distributions than monomer molecules. The same tendency has been found in the case of methyl iodide.31 Sapers et ~ 1 . have ~ ~ also 9 ~reported ~ on I2+ and I+ ion signals originated from clusters. In this study the I+ signals produced from the clusters and from the direct MPI process are observed in the center of T O F spectra, indicating that the signals do not show the laser polarization dependence. Therefore, we have minimized the contribution from clusters by choosing only the early part of the molecular beam pulse for photolysis. Figure 3a shows the TOF distributions, N*(TOF), of iodine ion produced a t 277.38 nm with polarization angles a = 0' and 90'. At this wavelength, the I* atom can only be ionized via REMPI. The two TOF peaks correspond to the iodine ions recoiling toward and away from the detector. The ion signal at a = 0' is ca. 4 times stronger than that at a = 90'. This suggests that the major I* fragments recoil along the direction of the electricvector of the laser light after a photon absorption. Figure 3b gives the TOF distributions of iodine ion measured at 277.87 nm, where the ground-state iodine atom I can only be ionized by REMPI. The intensity difference of I+ signals is even greater a t the two laser polarization angles as compared with the case of I*, suggesting that the angular distribution of the I atom is more anisotropic. In addition, the fact that the ion intensity decreases markedly on going from a = 0' to a = 90' is suggestive of a primary photodissociation process with a dominant parallel polarization dependence. The laboratory recoil velocity distribution F(v,B) of iodine
Kang et al.
1528 The Journal of Physical Chemistry, Vol. 98, No. 6, 1994
/
?-1O
0
200
..'
400
\
600
'b
800
I
1000 0
Figure 4. Recoil speed distribution g(u) and the anisotropy parameter B(u) of I* and I atoms obtained from the laboratory recoil velocity
distributions F(u,B).
fragment is obtained by transforming Nu(T0F) in Figure 3.
F(u,B) = W(TOF)(dTOF/du)
10
20
30
40
50
Et, kcal/ m o 1
V , m/s
(3)
where u is the photofragment recoil speed in the laboratory frame and B is the laboratory recoil angle with respect to the electric vector of photolysis light. The angular distribution as well as the energy partitioning between internal excitation and relative translation of the products affects the velocity component distribution and the peak shape. The F(u,B) distribution is expressed in the form.34-38 (4)
where B(u) is the recoil anisotropy parameter, g(u) is the recoil speed distribution, and PZ(COS e) = (3 cosz 8 - 1)/2 is the secondorder Legendre polynomial. The recoil velocity distribution is then characterized by g(u) and B(u). The /3 is constrained to lie in the range -1 I6 I+2, depending on the degree of prompt dissociation with the molecular transition dipole moment axis lying either perpendicular or parallel to the dissociation bond axis. The g(u) and @(u)for I* and I atoms are calculated using a deconvolution technique39and displayed in Figure 4. The g(u) distributions of I* and I atoms show the same feature with no significant structures. The recoil speed distributions of I* and I atoms are narrow and have their maxima a t high recoil speeds. These phenomena indicate that both iodine atoms are produced from repulsive potential energy surfaces which result in nonstatistical energy partitioning toward the translational mode. These are the typical characteristics of the electronic states of alkyl iodides in A absorption band. The O(u) of I* and I atoms in Figure 4 provide further details for the nature of electronic transitions. The /3 of the I atom is close to the limiting value of 2 for a pure parallel transition (transition dipole moment oriented parallel to the C-I bond) while that of the I* atom is substantially smaller. From eq 5 the average anisotropy parameter 6 is calculated
= (111 - 11)/(0.5Z1+ 11) (5) where 11 and ZI denote the integrated intensities of F(u,8) a t B = Oo and 90°, respectively. We obtain 6 = 0.9 f 0.1 for I* and = 1.6 f 0.1for I, consistent with the calculated value from the deconvolution method as mentioned above. The anisotropy parameter observed in the two dissociation channels is interpreted in accord with the known naturel1JZ of the three possible electronic transitions in the A band of alkyl iodides. Each transition must be polarized either parallel (3Q0) or perpendicular
Figure 5. Translational energy release distributions of I* and I formation channels at two recoil angles. The solid lines and the dots correspond to 0 = Oo and 90°, respectively. The distributions are normalized to have the unit peak height. The distributions show the average translational energies of 11.5 (for the I* channel)and 17.7 kcal/mol (for the Ichannei).
(3Ql and ~ Q I to ) the C-I bond axis. The dissociation time is much shorter than the rotation time of the parent m ~ l e c u l e . ~ ~ * The prompt dissociation of n-C4HgI is also evident from the observed narrow recoil speed distributions with high recoil speeds. Thus, the anisotropy parameter of each transition must lead to its limiting value, Le., +2 for the parallel and -1 for perpendicular transitions. Since the 6 of I is close to +2, the parallel transition to the 3Q~state is predominant. Therefore, the3Q0+Ntransition followed by curve crossing to the 'QI state, which dissociates into I, is found to be the dominant contribution over that of the 3Q1 N or IQI N transition. The observed 6 of I*, far lower than expected, shows a quite different behavior compared to other alkyl iodides studied to date. The angular distribution of I* from the photodissociation of alkyl iodides5J7vu2 has shown a strong anisotropic character due to the predominant contribution from the3Qo-Ntransition. This is explained by the formation of I* from more than one state. The observed of I* can then be attributed to mixed transitions of the parallel 3Q0 N a n d the perpendicular lQ1N . Alternatively, the decrease of the anisotropy parameter in each channel from its limitingvalueof +2 or -1 can be interpreted as the result from rotation of the parent molecule prior to dissociation. The rotation of dissociating molecules, however, may not be a primary source since the high anisotropic character for I at 304.67 nm is hardly explained by the rotational motion. The translational energy release distributions GB(Et)of the I* and I channels are obtained from F(u,B) by applying the conservationof linear momentumof photofragments at two recoil angles and are displayed in the normalized form of peak intensity in Figure 5 .
-
-
-
where Et is the total translational energy of the iodine atom and the C4H9 radical and mt and mR are the masses of the iodine atom and that of the C4H9 radical, respectively. The available energies, Eav1and P a v 1 for I and I* formation channels can be determined from the energy conservation relations:
Eavl= hv - D;
+ EPint= E, + Eint
for the I formation (7)
E*avl= Ea",- E , = E*, + E*int for the I* formation (8)
Photofragment Translation Spectroscopy of n-C4H91
The Journal of Physical Chemistry, Vol. 98, No. 6, 1994 1529
TABLE 2 Average Anisotropy Parameter and Partitioning of the Available Energy in the Photodissociation of RC&P iodine state & (nm)
( E d/Ea"] Eavl (El) (Ei,,) ex$ softc rigidd I* 277.38 0.9f0.1 33.5 11.5 22.0 0.66 0.72 0.49 I 277.87 1.6f0.1 55.1 17.7 37.4 0.68 0.72 0.49 I* 304.02 24.3 5.7 18.6 0.76 0.72 0.49 I 304.67 1.9*0.1 46.1 13.9 32.2 0.70 0.72 0.49 a Energies are in kilocalories per mole. (Eint)/Eavlmeasured in this work. (Eint)/Eavlimpulsive model calculated on the basis of the soft radical limit. (Eint)/Eavlimpulsivemodel calculated on the basis of the rigid radical limit.
8
v1
ze
1
z
A 0.54
Lrj
v
m
0
0.0-
I (304.67nm)
n
f(304.02nm)
1.0 -
$
I:
I'(304.02nm)
0
10
20
30
50
40
Et, k c a l / m o l Figure 7. Translational energy release distributions of I* and I formation channels at recoil angle of 0 = OD. The distributions are normalized to the unit peak height.
,
t 20.0
20.4
20.8
21.2
TOF, p s e c Figure 6. Time-of-flightdistribution of resonantly-ionized (a) I*(zP1/2) and (b) I(2Pap) atoms produced from photodissociation of n-C4HgI at 304.02 and 304.67 nm, respectively, with laser polarization angle a = .'0 The early and late componentsof the arrival time distribution correspond to the downward and upward velocity components of the iodine recoil fragments. The delay time of the repelling pulse was set at 7d = 0.5 ps. where hu is the photon energy, Doo the dissociation energy of n-C4HgI into the C4H9 radical and the I atom at 0 K (47.8 kcal/ mol), D i n t the internal energy of the parent molecule, and E,, the spin-orbit excitation energy of iodine atom (21.7 kcal/mol). Doo is obtained by subtracting the thermal energP2 from the bond dissociation energy a t 298 K.43 After dissociation, the available energy partitions into the internal energy (Eint)of the C4Hg radical and the total cm translational energy of the photofragments (E,). D i n t is neglected for the calculation assuming a monoenergetic molecular beam. The Eavland Elav,for two dissociation channels are indicated by vertical arrows in Figure 5 . The observed ( E , ) are summarized in Table 2 together with the available energy and the energy partitioning. The translational energy release distribution has much smaller width (fwhm I? 11 kcal/mol) in the I* formation channel than that for the I channel (fwhm N 14 kcal/mol), as shown in Figure 5. This is evidence that the internal energy left in the C4H9 radical is distributed more narrowly for the I* channel than the I channel. This behavior can be explained by the difference in the internal energy available in two channels. Since I* carries an extra 21.7 kcal/mol compared to I, the energy available for the radical becomes much smaller in this case. The larger the available energy, the more ways the energy can be distributed and the broader would be the internal energy distribution in C4H9 radical. (C) Photodissociation in the 304-nm Region. The TOF distributions of recoiling iodine atoms (Figure 6) for n-CdH91 photolysis in the 304-nm region allow us to assess the nature of the excited states leading to the absorption profile and the internal energy distributions. The early and late components of the arrival time distribution correspond to the downward and upward velocity components of neutral recoiling fragments. In this study, we
were able to detect the small amount of I* atoms (only at a = Oo polarization angle), providing clear evidence that the I formation process dominates in the long wavelength end of the A absorption band. A similar trend has also been found in other ~ y s t e m s ,in~ ~which ~ ~ the I*/I branching ratios from the photodissociation of CFzIz and CHzIz molecules exhibit a steep increase of the ground-state I atoms with a decrease of photon energy. TheP(1)at 304.67 nm, 1.9 f0.1,isobtainedfromtheintegrated intensities a t two polarization angles. In the case of I* at 304.02 nm, although the ion signals at a = 90° are too weak to obtain thequantitative&I*), they are strong enough to see the I* channel has also a strong anisotropic character of parallel transition. The p(I) (=1.9fO.l)at 304.67nmisfoundtobelargerthanthe&I) (=1.6 f 0.1) at 277.87 nm. A similar case has been found in i o d ~ e t h a n e sphotolysis ~ ~ . ~ ~ at 308 nm; i.e.,sole ground-state iodine atoms formed with parallel polarization. The large anisotropy parameter of I at 304.67 nm confirms that the perpendicular transition to the IQIstate does not contribute to the formation of I as the excitation energy shifts to the longer wavelength. The Go@,) of I and I* channels a t 6 = Oo are displayed in Figure 7. It is noteworthy that the most probable E, values are now shifted to the low-energy region in comparison with the result at 277-nm photolysis. There is marked difference in the peak widths between two excitation wavelengths, 277 and 304 nm. The translational energy release distribution of the I channel (fwhm N 10.6 kcal/mol) a t 304.67 nm is substantially narrower than those (fwhm 14.0 kcal/mol) a t 277.87 nm (see Figure 5 ) . A quantitative analysis of this difference indicates that the smaller the available energy, the narrower would be the width of internal energy distribution in the C4H9 radical fragment, resulting in the narrow range of energy partitioning caused by the photon energy. IV. Discussion
-
A quasidiatomic picture,I2 which assumes the u* n transition is localized on the C-I bond, predicts ground-state iodine formation arising from a perpendicular transition to 3Q1 or 'QI states and spin-orbit excited-state iodine arising from a parallel transition to a 3Q0state. Within the context of this picture, the observed angular dependence of both the ground- and excited-state iodine channels reveals that a predominantly parallel transition (3Q0 state) contributes in the photodissociation of n-CdHgI. In the 277-nm region, I* atoms are produced mainly via the parallel transition to the 3Q0state, whereas I atoms are formed predominantly by initial excitation of the 3Q0 state followed by a nonadiabatic curve crossing to the 'Q1 state. The parallel anisotropic character for the I formation channel = 1.6)
(s
Kang et al.
1530 The Journal of Physical Chemistry, Vol. 98, No. 6, 1994 TABLE 3: Curve Crossing Probability and Quantum Yield in the Photodissociation of mCJiJ A,, dissociation iodine quantum crossing (nm) transition path state yield probability I* 0.47 277 parallel, direct 0.43 0.36 'Qo N curve crossing I I 0.03 perpendicular, direct 0.14 0.82 curvecrossing I* 'Q1-N I* 0.14 304 parallel, direct 0.86 0.86 'QO N curve crossing I
where V12is the coupling term between the diabatic potential, AF is the difference in potential slopes at the crossing point, and u is the velocity through the crossing point. The Landau-Zener parameter {includes V12 and AFterms and is constant for a given molecule. In the case at hand, the probability of curve crossing depends only on thecrossing velocity u. Assuming that thecrossing velocity corresponds to the final velocity of the separating photofragments, u is calculated from
suggests that the transition dipole moment lies predominantly parallel to the C-I bond axis and the excited state breaks up on a short time scale compared to a rotation period of the parent molecule. It is interesting to note that the I* formation channel for the n-C4H91molecule shows a very different behavior when compared to other alkyl iodides studied to date. The angular distribution of I* at 277-nm photolysis is observed to be substantially less anisotropic (p = 0.9) than that for I (p = 1.6). Considering the predicted polarization dependence of the three electronic states of the alkyl iodide A band, this contradiction is explained by proposing that I* atoms originate from a mixed transition. Although the majority of I* atoms are produced by parallel transition to the 3Q0 state, there is still significant intensity at CY = 90'. A likely explanation for the ion signal at CY = 90' relies on I* production by a curve crossing mechanism. The I* atoms that are ejected at CY = 90' must be produced from the initial excitation of parent molecules to 'QI or 3Ql states. The highly anisotropic character of I atoms, however, precludes the possibility of the 3Q1transition which correlates with the ground-state iodine product channel. Since the lQ1 state also correlates with the I formation channel, I* atoms ejected at CY = 90' can only be produced from the perpendicular transition to the IQl state followed by a curve crossing to the 3Q~state during the dissociation, as suggested by Chandler et in a recent study of CH3Br system. The relative contribution of each transition in a mixed 3Q0and IQ1 transition to I* and I formation channels can be determined by using the simple relations
where ( E , ) is the experimental average translational energy of the photofragments. Therefore the less energy released in translation during photodissociation processes, the slower is the passage through the crossing region and the greater is the chance of curve crossing between the two excited states. As summarized in Table 2, our observation shows that the translational energies of photofragments at 304 nm are considerably smaller than those of photofragments at 277 nm. The extensive formation of a ground-state iodine component at 304 nm is now interpreted as arising from a slow velocity through the curve crossing, thereby increasing the curve crossing probability (see Table 3). These simple considerations indicate that the excitation photon energy plays an important role in the curve crossing mechanism. Another interesting feature to be noted is that the parameter {is characterized by the potential energy surface of the molecule and is thus independent of excitation wavelength. On the basis of the measured curve crossing probability P and average translational energy of photofragments ( E , ) , one can estimate parameter {. { values obtained for the 3Q0 N transition are 910and2170m/sat 277and304nm,respectively. Thedifferent { values for the same molecule clearly indicate that the true crossing velocity greatly, differs from the final velocity of photofragments. This discrepancy is believed to be due to the assumption that the curve crossing position is far enough out along the C-I coordinate and that the crossing velocity thus can be considered as the final velocity of photofragments, as suggested by Godwin et aI.I9 From a recent study on potential energy surface of CH31,49however, the C-I distance at the curve crossing point was calculated to be 2.35 A, which is close to the groundstate equilibrium distance of 2.14 A. The calculated potential energy a t the crossing point is 3 1 100 cm-I, compared to the final asymptotic energy (24 900 cm-I) of the I* dissociation channel. If the curve crossing occurs on the strongly repulsive part of the potential energy surface of n-C4H9I, then the crossing velocity would be smaller than the final velocity of the photofragments. This overestimation of the crossing velocity in the calculation of the { parameter is pronounced as the exciting photon energy decreases, thereby producing the large (value. Compared to the obtained { (=910 m/s) value at 277 nm, which is comparable to the result of Godwin et al.I9 on n-alkyl iodides at 248 nm ({ = 1100 m/s), it is clear that the { (=2170 m/s) value at 304 nm produces a significant deviation due to the overestimation of the crossing velocity. In view of the present results, one can conclude that the true position of curve crossing has a profound effect on the curve crossing probability. As observed for small alkyl iodides, the translational energy release has been measured to be very high, reflecting the nonstatistical behavior of dissociation on the repulsive 3Q0 state. In the photodissociation of n-CdHgI, however, only about 30% of the available energy is released in translation (see Table 2) a t the two wavelengths studied. Thus we see that the long alkyl structure produces a quite inefficient energy disposal to the translational motion of photofragments. In addition, the translational energy extends to substantially lower values as photolysis wavelength increases. This photon energy dependenceof energy partitioning reflects the shape of the 3Q0 potential energy surface and the excess energy dependence of the nonradiative energy distribution in 3Q0 state.
+
-
In eq 9, Nl,(I*) and N,(I) represent the numbers of I* and I atoms produced by direct dissociation from the excited states 3Q0 (parallel) and IQl (perpendicular), respectively. N,(I*) and Nil(I) also correspond to the numbers of those atoms but are produced from the two transitions followed by a curve crossing between these states. Using eqs 2, 9, and 10, we obtained the relative quantum yields of I* and I atoms according to their dissociation paths. The results are summarized in Table 3. As is generally found for the photolysis of alkyl iodides, the photodissociation of n-C4H9I carries a small contribution of the lQ1 state (17%) at 277 nm, whereas only a parallel transition to the 3Q0 state is responsible for dissociation processes at 304 nm. In addition, th'e curve crossing between two excited states plays an important role in I* and I formations and become more prominent in the longer excitation wavelength (304 nm). The nature of curve crossing between two excited states has been explained qualitatively with a simple Landau-Zener model.19 The probability of curve crossing is given by
P = 1 - exp(-2?rV12/hlAl+) = 1 - exp(-t/u)
(11)
-
Photofragment Translation Spectroscopy of n-C4H91 The fraction of the available energy which goes into internal excitation of the alkyl fragment is about 70% for the photodissociation of n-C4H9I. This fraction is in good agreement with the increasing propensity for internal excitation of alkyl iodides (from 17% for CH31to 54% for n-C3H71) which is measured by Zhu et a1.18using a rotatable pulsed molecular beam and quadrupole mass filter. The trend toward increased internal excitation with the size of the alkyl radical is indicativeof the structuredependence on the dynamics of molecular processes. The present results, when compared with statistical and impulsive dynamic models, indicate that the photofragmentation of n-C4H91 is consistent with the direct impulsive model of unimolecular decomposition. Hence the photodissociation of n-C4H91, similar to small alkyl iodides, should proceed on a strongly repulsive surface along the C-I bond with a short lifetime of about s.4* The extent of the translational energy release can be explained by comparison with the predictions of two limiting impulsive models1 for photodissociation dynamics of alkyl iodides. The “rigid” radical limit assumes the alkyl radical as a rigid body so that only rotational excitation can occur during fragmentation. In this limit no vibrations of the radical are excited, and the maximum possible recoil energy is achieved. The available energy is distributed solely between product recoil energy and rotational energy of the polyatomic radical. In the “soft” radical limit, the a-carbon atom is assumed to be so weakly attached to the rest of the alkyl fragment in relation to the sharp C-I repulsion that it alone initially absorbs the energy as the fragments repel one another. The a-carbon atom then recoils into the rest of the radical, exciting the vibrational and rotationaldegrees of freedom of the radical fragment. The partitioning of energy between translation and internal excitation is decided by simple conservation of energy and momentum. In this way the soft radical limit gives the minimum possible recoil energy for a given available energy. The ( Eint)/Eavl values calculated from each model are summarized in Table 2. The experimental results show that the photodissociation of n-C4H9I in both I* and I channels is close to thesoft radical limit. Therefore, the bond between the a-carbon and iodineatom is very soft, resulting in highvibrational excitation in the butyl radical. It is yet not possible to unravel the various factors which might contribute to the electronic transitions and the curve crossing due to the lackof dynamic information available on the n-CdH91 molecule.
Acknowledgment. This work was supported from the Ministry of Science & Technology under the Atmospheric Pollution Control Project and from the Korea Research Institute of Standard & Science, which is gratefully acknowledged. References and Notes (1) Riley, S.; Wilson, K. R. Discuss. Faraday SOC.1972, 53, 132.
The Journal of Physical Chemistry, Vol, 98, No. 6,1994 1531 (2) van Veen, G. N. A.; Baller, T.; de Vries, A. E.; Van Veen, N. J. A. Chem. Phys. 1984,87,405. (3) Barry, M. 0.;Gorry, P. A. Mol. Phys. 1984, 52, 461. (4) Minton. T. K.: Felder.. P.:. Brudzvnski. R. J.: Lee. Y. T. J . Chem. Phys. 1984, 81,‘1759. ( 5 ) Ogorzalek Loo, R.; Hall, G. E.; Haerri, H.-P.; Houston, P. L. J . Phys. Chem. 1988, 92, 5. (6) Felder, P. Chem. Phys. 1991, 155, 435. (7) Dzvonik, M.; Yang, S.; Bersohn, R. J. Chem. Phys. 1974,61,4408. (8) Knee, J. L.; Khundkar, L. R.; Zewail, A. H. J . Chem. Phys. 1985, 83, 1996. (9) Kasper, J. V. V.; Pimental, G. C. Appl. Phys. Lett. 1964, 5 , 231. (10) Bosch, R. A.; Salhub, D. R. Mol. Phys. 1972, 24, 289. (1 1) Mulliken, R. S.J . Chem. Phys. 1935, 3, 506. (12) Mulliken, R. S. J . Chem. Phys. 1940, 8, 382. (13) Gedanken, A.; Rowe, M. D. Chem. Phys. Lett. 1975, 34, 39. (14) Gedanken, A. Chem. Phys. Lett. 1987, 137,462. (15) ShaDiro. M.: Berson. R. J . Chem. Phvs. 1980. 73. 3810. (16j Spaik, R.K.; Shobatake, K.; Carlson, L. R.; Lee; Y. T. J . Chem. Phys. 1981, 75, 3838. (17) Penn, S. M.; Hayden, C. C.; Carlson Muyskens, K. J.; Crim, F. F. J. Chem. Phys. 1988,89, 2909. (18) Zhu, Q.;Cao, J. R.; Wen, Y.; Zhang, J.; Huang, Y.; Fang, W.; Wu, X . Chem. Phvs. Lett. 1988. 144.486. (19) God;in,F. G.;Gorry,P.A.;Hughes,P. M.;Raybone,D.; Watkinson, T. M.; Whitehead, J. C. Chem. Phys. Lett. 1987, 135, 163. (20) Brewer, P.; Das, P.; Ondrey, G.; Berson, R. J . Chem. Phys. 1983,79, 720. (21) Jung, K. W.; Choi, S. S.; Jung, K.-H. Rev. Sci. Instrum. 1991, 62, 2125. (22) Donovan, R. J.; Flood, R. V.; Lawley, K. P.; Yencha, A. J.; Ridly, T. Chem. Phys. 1992, 164,439. (23) Hwang, H. J.; El-Sayed, M. A. Chem. Phys. Lett. 1990, 170, 161. (24) Hwang, H. J,.; El-Sayed, M. A. J. Chem. Phys. 1991, 94, 4877. (25) Jiana. Y.: Giorai-Armazzi, M. R.; Bernstein, R. B. Chem. Phys. 1986, i06, 171. (26) Tamres, M.; Duerksen, W. K.; Goodenow, J. M. J . Phys. Chem. 1968, 72, 966. (27) Passchier, A. A,; Gregory, N. N. J . Phys. Chem. 1968, 72, 2697. (28) Clear, R. D.; Wilson, K. R. J. Mol. Spectrosc. 1973, 47, 39. (29) Hwang, H. J.; El-Sayed, M. A. J . Phys. Chem. 1991, 95, 8044. (30) Hunter, T. F.; Kristjansson, K. S. Chem. Phys. Lett. 1978,58,291. (31) Ogorzalek Loo, R.; Haerri, H.-P.; Hall, G. E.; Houston, P. L. J . Chem. Phys. 1989, 90,4222. (32) Sapers, S.P.; Vaida, V.; Naaman, R. J . Chem. Phys. 1988,88,3638. (33) Vaida, V.; Donaldson, D. J.; Sapers, S. P.; Naaman, R. J . Chem. SOC.,Faraday Trans. 1990,86, 2043. (34) Zare, R. N.; Herschbach, D. R. Proc. IEEE 1963, 51, 173. (35) Zare, R. N. Mol. Photochem. 1972, 4 , 1. (36) Busch, G. E.; Wilson, K. R. J . Chem. Phys. 1972,56, 3638. (37) Yang, S.; Bersohn, R. J . Chem. Phys. 1974,61, 4400. (38) Hwang, H. J.; El-Sayed, M. A. J . Phys. Chem. 1992, 96, 8728. (39) Hwang, H. J. Ph.D. Thesis, University of California, Los Angeles, 1991. (40) Felder, P. Chem. Phys. Lett. 1992, 197, 425. (41) Paterson, C.; Godwin, F. G.;Gorry, P. A. Mol. Phys. 1987,60,729. (42) Godwin, F. G.; Peterson, C.; Gorry, P.A. Mol. Phys. 1987,61,827. (43) McMillen, D. F.; Golden, D. M. Annu. Rev. Phys. 1982, 33, 493. (44) Koffend, J. B.; Leone, S. R. Chem. Phys. Lett. 1981.81, 136. (45) Baum, G.; Felder, P.; Huber, J. R. J. Chem. Phys. 1993,98, 1999. (46) Minton, T. K.; Nathanson, G. M.; Lee, Y. T. Laser Chem. 1987.7, 297. (47) Minton, T. K.; Nathanson, G. M.; Lee, Y. T. J . Chem. Phys. 1987, 86, 1991. (48) Hess, W. P.;Chandler, D. W.; Thoman, J. W., Jr. Chem. Phys. 1992, 163, 277. (49) Tadjeddine, M.; Flament, J. P.; Teicheil, C. Chem. Phys. 1987,118, 45.
