Polarization of cyanogen(B2.SIGMA. -X2.SIGMA. ) emission

---

2916

“=(k+l)

J. Phys. Chem. 1985,89, 2916-2918

-1

k2 At 175 “C, the thermal decomposition of alkyl nitrites proves to be a convenient source of alkoxy radicals. Batt has shown previously that the decompositions of the alkyl nitrites are independent of the alkyl group and typically have the same Arrhenius parameters.* However, the decomposition (and in some cases the isomerization) of the alkoxy radical does depend on the alkyl group, and some would be expected to decompose at 175 OC. Batt has shown that straight-chain alkoxy radicals can decompose primarily by two pathways RO

+H CH2O + R RCHO

-+

-+

(3a) (3b)

If reaction 3a occurs to an appreciable extent, our aldehyde rate measurement would be higher than expected and cause considerable problems with the analysis. Fortunately, previous studies have shown reaction 3b to be the sole fate of straight-chain R O decomposition reactions a t 175 O C . * Thus, all the aldehyde produced in these systems is produced from reaction 2b and not from the decomposition of the alkoxy radical. This is confirmed experimentally by the fact that the computed values for kZb/k2 are independent of N O pressure. Also, the isomerization reactions that can occur with the n-C4H90 radical would not be expected to produce n-C3H7CH0.6*9 From an inspection of the results in Table I1 the values of kzb/kz in all cases are independent of [NO],. This would be expected since we are only interested in the relative rates of both processes. For the C 2 H 5 0and n-C3H70systems we were able to measure both rates at relatively low [NOIT. However, for the i-C4H90 and n-C4H90 systems we were unable to measure these rates at low [NOIT and therefore had to start a t = l o torr of [NO],, because of the fast decomposition (or isomerization) of these radicals relative to the R O ISNO reaction. We can conclude that k2,/k2 = 0.22 f 0.02,0.26 f 0.03,0.29 f 0.05, and 0.33 f 0.03, respectively, for C z H 5 0 , n-C3H70, n-C4H90, and i-C4H90. The uncertainties represent estimated

+

(8) L. Batt, Znt. J . Chem. Kinet., 11, 977 (1979), and references therein. (9) A. C. Baldwin and D. M. Golden, Chem. Phys. Lerr. 60,108 (1978).

TABLE III: Primary Quantum Yields of RONO at 366 nm RONO k2blk2‘ 6ikdk2 41 rep C2HsONO 0.22 f 0.02 0.070 f 0.006 0.32 f 0.04 4 n-CpHTONO 0.26 f 0.03 0.115 f 0.010 0.44 f 0.06 5 n-CdH90NO 0.29 f 0.05 0.054 f 0.005 0.19 f 0.04 6 i-C4H90N0 0.33 f 0.03 0.064 f 0.005 0.19 f 0.02 7 “This work. *For absolute errors. Furthermore, an apparent trend exists between k2,/k2and the molecular complexity. As the molecular complexity increases, the value of k2,/k2 increases. It is not clear why this occurs. The values are independent of temperature, so the effect is not caused by an activation energy. Our k2,/k2values for C 2 H S 0and n-C3H70agree with the literature values presented in Table I within the experimental uncertainty. Finally, since the determination of the primary quantum yield of the photolysis of alkyl nitrites has been hampered by inherent problems of the techniques employed,I0 the values for k2,/k2 determined in this study can be coupled with ratios of kZb&/k2 determined in other studiese7 to calculate the primary quantum yield, c#J~. The calculated values of are presented in Table 111.

Acknowledgment. This work was supported by the Center for Air Environment Studies at Penn State University for which we are grateful. Registry No. C2HSON0, 109-95-5; n-C3H70N0, 543-67-9; nC4H90N0,544-16-1; i-C4H90N0, 542-56-3; C2H50,2154-50-9; nC3H70,16499-18-6; n-CdH90, 19062-98-7; i-C4H90,26397-34-2; 15N0, 15917-77-8. (10) P. Morabito and J. Heicklen, Int. J . Chem. Kinet.,in press. (11) H. A. Wiebe and J. Heicklen, J. Am. Chem. SOC.,91, 1085 (1969). (12) P. Gray and M. W. Pratt, J . Chem. SOC.,3403 (1958). (13) R. L. East, J. R. Gilbert, and L. Phillips, J . Chem. SOC.A , 1673 (1968). (14) R. E. Rebbert, J . Phys. Chem., 67, 1923 (1963). (15) R. A. Livermore and L.Phillips, J . Chem. SOC.E , 640 (1966). (16) B. E. Ludwig and G. R. McMillan, J . Am. Chem. SOC.,91, 1085 (1969). (17) G. R. McMillan, J . Am. Chem. Soc., 83, 3018 (1961). (18) G. A. Hughes and L. Phillips, J. Chem. SOC.A, 894 (1967). (19) B. E. Ludwig and G. R. McMillan, J . Phys. Chem., 71,672 (1967). (20) M. J. Yee Quee and J. Thynne, Tram. Faraday SOC.,64, 1296 ( 1968). (21) R. L. East and L. Phillips, J . Chem. SOC.A , 331 (1970). (22) R. F. Walker and L. Phillips, J . Chem. SOC.A , 2103 (1968). (23) R. L. East and L. Phillips, J . Chem. SOC.A , 1939 (1967).

Polarizatlon of CN(B2z+-X2z+) Emission Produced in Collision of Ar(3P,,,) with BrCN Takashi Nagata, Tamotsu Kondow,* Kozo Kuchitsu, Department of Chemistry, Faculty of Science, The University of Tokyo, Bunkyo-ku, Tokyo 113. Japan

Kiyohiko Tabayashi, Shigeru Ohshima, and Kosuke Shobatake Institute for Molecular Science, Myodaiji, Okazaki 444, Japan (Received: January 2, 1985)

Rotational alignment in the CN(B22+)fragment was observed by measuring polarization of the CN(Bz2+-X28+) emission from BrCN excited by Ar(3Po,z)impact. The degree of polarization with respect to the beam axis was 0.023 & 0.004 at a collision energy of 1.33 eV and decreased with the impact energy. This observation indicates that the production of CN(B) is caused primarily by energy transfer from Ar(3P,,2) to BrCN.

Introduction The CN(BZz+-XzE+) emission produced by Vacuum-UV photodissociation or electron-impact dissociation

hu or c

-

BrCN [BrCN]* Br + CN(B22+) (1) has been studied extensively. This emission is found to be polarized

with respect to the direction of the electric vector, c, of the photons or the direction Of the momentum vector, k, of the impinging electrons.’-“ This polarization originates from the fact that the (1) G. A. Chamberlain and J. P. Simons, J . Chem. SOC.,Faroday Trans. 2, 71, 2043 (1975).

0022-3654/85/2089-2916$01.50/00 1985 American Chemical Society

The Journal of Physical Chemistry, Vol. 89, No. 13, 1985 2917

Polarization of CN(B22+-XZZ+)Emission parent linear molecule is excited to the precursor state most efficiently when this molecule is oriented in a certain direction relative to the direction o f t or k.s,6 Analyses of the degree of polarization have provided information on the symmetry and lifetime of the precursor state, [BrCN]*, and the momentum distribution of the scattered electrons. The present study reports on a measurement of the polarization of the CN(B-X) emission produced by7-I0

+

Ar(3Po,2) BrCN

-

Ar('So)

+ Br + CN(B)

(2)

This emission is expected to be partially polarized with respect to the direction of the relative velocity of the reactants, because the energy transfer from the argon atom to BrCN should occur efficiently when the reactants collide with each other along a certain preferred trajectory. In addition, the magnitude of the polarization may vary with collision energy because the angular momentum distribution of the excited molecules should depend on the energy. Hence, the measurement of the polarization should contribute to elucidation of the excitation process in question. This measurement requires a beam of metastable atoms which has restricted divergence and variable kinetic energy. In the present study, an intense Ar(3Po,2)beam produced by the method of an arc-heated supersonic nozzle beam" was used to measure the degree of polarization of the CN(B-X) emission in process 2 as a function of the collision energy ranging from 1.2 to 2.1 eV. Recently, Simons and co-workers generated a fast beam of metastable rare-gas atoms by means of a rotor-accelerated beam.l2 They applied this technique to the Xe(3Po,2)/BrCNsystem in the range of collision energy between 0.07 and 0.8 eV. They showed that the CN(B-X) emission had no measurable polarization, and on this basis they concluded that the CN(B) was produced through a [Xe+CN-]* c0mp1ex.l~ Formation of such a complex is one of the possible mechanisms proposed by Setser et al. for the CN(B) production in the reaction of metastable rare-gas atoms, Rg(3Po,2),with cyanogen halides, XCN: (1) In the "energy-transfer" mechanism, an excited [XCN]* precursor dissociates to produce CN(B), as in the case of photodissociation or electron-impact dissociation;' (2) in the "reactive" mechanism, an [Rg+CN-]* complex is formedS9N o clear understanding has yet been given as to which mechanism is dominant in the Ar(3Po,2)/BrCNsystem. The polarization and its collision energy dependence measured in the present study are used for discussion of these mechanisms with the aid of the studies on the polarization of chemiluminescence by Zare et al.I4*l5and by Simons et a1.'6-20 (2) M. T. Macpherson and J. P. Simons, J. Chem. SOC.,Faraday Trans. 2, 75, 1572 (1979). (3) M. N. R. Ashfold, A. S.Georgiou, A. M. Quinton, and J. P. Simons, J. Chem. SOC.,Faraday Trans. 2, 77, 259 (1981). (4) T. Nagata, T. Kondow, and K. Kuchitsu, Chem. Phys. Lett. 95, 97 (1983). ( 5 ) M. T. Macpherson, J. P. Simons, and R. N. Zare, Mol. Phys., 38,2049 (1979). (6) T. Nagata, T. Kondow, and K. Kuchitsu, Chem. Phys., 72,281 (1982). (7) J. A. Coxon, D. W. Setser, and W. H. Duewer, J. Chem. Phys., 58, 2244 (1973). (8) T. Urisu and K. Kuchitsu, J. Photochem., 2, 409 (1973-1974). (9) D. W. Setser, T. D. Dreiling, H. C. Brashears, Jr, and J. H. Kolts, Faraday Discuss. Chem. SOC.,67, 255 (1979). (10) A. J. Yencha, Y. Ozaki, T. Kondow, and K. Kuchitsu, Chem. Phys., 51, 343 (1980). (11) K. Shobatake and K. Tabayashi, At. Collision Res. Jpn., 8, 157 (1982). (12) P. B. Moon, C. T. Rettner, and J. P. Simons, J. Chem. Soc., Faraday Trans. 2, 74, 630 (1978). (13) R. J. Hennessey, Y. Ono, and J. P. Simons, Chem. Phys. Lert., 75, 47 (1980). (14) C. D. Jonah, R. N. Zare, and Ch. Ottinger, J . Chem. Phys., 56,263

.--(15) M.G.Prisant, C. T. Rettner, and R,N. Zare, J . Chrm. Phys., 75,

11972). -,.

2222 (1981). (16) C. T. Rettner and J. P. Simons, Faraday Discuss. Chem. SOC.,67, 329 (1979). (17) C. T. Rettner and J. P. Simons, Chem. Phys. Lett., 59, 178 (1978). (18) R.J. Hennessy and J. P. Simons, Chem. Phys. Lett. 75,43 (1980).

l&---+ 1.0

1.5

2.0

EJeV

Figure 1. Degree of polarization of the CN(B22+-X2Z+) emission plotted against the average collision energy of the reactants. The corresponding values for (U.2l2)are shown on the right-hand ordinate. Error bars represent one standard deviation (see text). The data without error bars, determined by a single measurement, are less accurate than the others.

Experimental Section Details of the experimental apparatus have been described elsewhere." The Ar(3Po,2)beam was produced by a plasma arc heater; arc-heated argon gas was expanded into a vacuum chamber through a supersonic nozzle of 0.7-mm diameter. The average collision energy was varied over 1.2-2.1 eV by changing the arc current (40-80 A) and the magnet current (0-10 A) with a stagnation pressure of 1 atm.21 The beam was collimated by a skimmer (1-mm diameter) with a nozzleskimmer distance of 20 mm and by a slot (3 mm X 5 mm) located at 95.5 mm from the skimmer. The beam divergence was restricted to 53' by this collimation. The Ar(3Po,2)beam was modulated by a chopper at a frequency of 50 Hz. The BrCN target gas stored at 100-200 torr was introduced through a nozzle of 0.1-mm diameter. The gas beam was crossed by the Ar(3P02) beam =16 mm downstream from the slot. The CN(B-X) emission was collected by a pair of lenses (f= 80 mm) in a direction orthogonal to both beams and was allowed to pass through a sheet polarizer (Polaroid HNP'B) and a polarization scrambler into a Spex 1870 0.5-m grating monochromator. The slits of the monochromator were set to be 600 pm, which corresponds to the spectral resolution of ~ 1 . nm 0 centered at 387.5 nm. This resolution covered the spectral range of the 0-0 and 1-1 bands of the CN(B-X) emission. The emission was detected by a Hamamatsu R585 photomultiplier, whose output signals were registered in a bidirectional counter. The signals and noises, measured alternatively in the modulation periods, were separately processed by a Fujitsu FM-8 microcomputer and stored into a floppy disk. The noises, which mainly came from the dark current of the photomultiplier, were subtracted from the total counts, and real photon signals were obtained. The polarization of CN(B-X) emission was measured with reference to the direction of the incident Ar(3Po,z)beam, since the beam-gas condition was approximately fulfilled under the present experimental c o n d i t i ~ n s ; ' ~that J ~ *is, ~ ~(1) the BrCN gas beam was more widely spread than the Ar(3Po,2)beam and (2) the velocities of the Ar(3Po,2),atoms were 1 order of magnitude greater than those of the target gas molecules. The degree of polarization was defined as (3) where 11, and I , denote the emission intensities polarized parallel and perpendicular to the incident beam axis, respectively. The procedure for the polarization measurement was as follows.6 The sheet polarizer was set for the measurement of Ill for 20-60 s and (19) R. J. HCMWY,Y.Ono, and J. P. Simons, Mol. Phys., 43, 181 (1981). (20) K.Johnson, R. Pease, and J. P.Simons, Mol. Phys., 52,955 (1984).

(21) The distribution of the atomic velocities in the beam was found to

obey the theoretical velocity distribution function given by Anderson et al. [J. B. Anderson .and J. B. Fenn, Phys. Fluids, 8, 780 (1965)l. In the present study, the average collision energy is given by E, = I/@:, where u, represents the stream flow velocity estimated by TOF measurements and I./ is the reduced mass of the reactants.

Nagata et al.

2918 The Journal of Physical Chemistry, Vol. 89, No. 13, 1985

then rotated by 90° for the measurement of I , . This procedure was repeated 40-100 times until the counts for each polarization component were accumulated to 4 0 000, and the degree of polarization was evaluated by eq 3.

Results and Discussion Figure 1 shows the degree of polarization measured as a function of the collision energy. The degree of polarization has a positive sign in the entire energy range studied. It is 0.023 f 0.004 at 1.33 eV and decreases gradually with increasing collision energy to 0.01 1 f 0.002 at 2.1 1 eV. The error represents a standard deviation estimated from 2 to 6 measurements at each collision energy. Generally, polarization originates from an anisotropic spatial distribution of the electric dipole oscillators that are responsible for the observed emission. In a classical treatment, the electric transition dipole, p, can be regarded as a Hertzian dipole fixed to the molecular frame; for example, in the case of a diatomic molecule, p lies either along the internuclear axis or perpendicular to it. Thus, the degree of polarization of emission can be related to the rotational alignment in the emitting molecules. In order toAd-acribethe rotational alignment, it is convenient to introduce ( U-ZI2),which is the average cosine square of the angle between the angular momentum vector, j, of the CN(B) product and the beam axis, Z. Here a unit vector is denoted by a car$. The degree of polarization is then expressed in terms of ( Ij.Zl2) as14*1s p = (1 - 3 (lj*212))/(3-

(fi*2I2))

(4)

where we use the knowledge that the CN(BZZ+-X2Z+) system is of a parallel-type transition; Le., p lies along the molecular axis. and It is readily shown from eq 4 that p varies fro? -1 to The vanishes for an isotropic angular distribution, ( kZl2)= (fi-gl*)values were estimated by use of eq 4 from the observed p values, as shown in Figure 1. In summary, (i) the angular momentum vectors of the product CN(B) are slightly aligned in the direction perpendicular to the incident beam axis (( @ZI2)< 1 / 3 and (ii) this angular distribution approaches_ ?n isotropic distribution as the collision energy is increased (( U.Z12) I / & . The j alignment and its collision energy dependence observed in the present study can be interpreted qualitatively in terms of the "energy-transfer" mechanism7

-

+

- + - + - + - +

Ar(3Po,2) BrCN [ BrCN] *

Ar('So)

Br

[BrCN]*

CN(B)

(5a) (5b)

rather than the "reactive" mechanismg

+

Ar(3Po,2) BrCN [Ar+CN-]*

Br

Ar('So)

[Ar+CN-]* CN(B)

(64 (6b)

no polarization is expected for the product emission because the incipient C N product should rotate freely in the dissociating [Ar'CN-] * complex and, consequently, the direction of j has no correlation with that of J. Instead, if a "tight-coupling" [Ar+CN-] * complex23is formed in process 6a, the anisotropy in the J distribution should be transformed into j alignment because of the angular correlation between J and j. If this is the case, the polarization of the product emission, if any, should increase with increasing collision energy because the J alignment is expected to occur more effectively at larger collision energy.l6-I9 These inferences conflict with either of the present findings (i) and (ii). On the other hand, findings (i) and (ii) can be explained by the energy-transfer mechanism, process 5 , as follows. In an energy-transfer process such as Kr(3Po,2)+ Br2 Kr(lSo) + Br2* (7)

-

the rotation vector of the electronically excited Br2* is reported to be preferentially polarized perpendicular to the direction of the relative velocity vector of the reactants, and this rotational alignment tends to be diminished with the increase in the collision energy.Is If the energy-transfer mechanism is operative in the present case, it is expected that the rotational angalar momentum of [BrCN] *, J, is preferentially directed perpendicular to the incident beam axis and tends to be distributed randomly with increasing collision energy. This anisotropy in the J alignment will be partially preserved in the product angular distribution after the dissociation of the [BrCN]* precusor (process 5b). In the limit of direct dissociation, the dissociating [BrCN]* is expected to retain the linear geometry of BrCN in the electronic ground state. In the case of predissociation, the [BrCN]* is likely to possess a character of the Rydberg states located around 10 eV24 and, hence, inherit a linear equilibrium geometry from the BrCN+ core ion.25 Therefore, the separating fragments carry off little orbital angular momentum, 1, during the dissociation. In addition, the magnitudes of J are expected to be much larger than those of 1 since electronic energy transfer from a metastable atom often excites a target molecule into a highly excited rotational Under these circumstances, the magnitudes and directions of J are partitioned into those of j and I in such a way that the average direction of j is along J, because IJI >> 111 = 0 and J = j + 1. Consequently, after the dissociation the distribution of the axis of fragment rotation should have a preferred direction perpendicular to the beam axis. The level of the final j alignment is probably lower than that of the J alignment because of the spread of the j distribution around J. In addition, it should depend on the collision energy as a result of the change in the J alignment with the collision energy. As discussed above, the present observation of the partially polarized emission and its collision energy dependence leads us to conclude that the energy-transfer mechanism 5a and 5b is a dominant process of the CN(B) production. This interpretation of the present experimental results is also favored by the internal energy distributions of the product in analogous processes28~29 Ar(3Po,2) H C N / D C N Ar(ISo) + H / D CN(B) (8)

-

The present discussion is based on the assumption that the reaction of Ar(3PoJ)with BrCN is characterized by these two mechanisms. Either of the mechanisms is not exclusive but is taken as an extreme case of reaction dynamics.13 and Under these assumptions, it can be inferred from the following Xe(3Po,2) ClCN/BrCN Xe('So) + Cl/Br + CN(B) (9) reasoning that the reactive mechanism does not contribute domand also by those in the present Ar(3Po,,)/BrCN system, which inantly to the production of CN(B). If the reactive mechanism will be discussed in a subsequent p u b l i ~ a t i o n . ~A~ further inoperates, the [Ar'CN-] * complex formed in process 6a should preferentially rotate about an axis perpendicular to the beam vestigation aiming at a more quantitative interpretation of the observed polarization values is in progress. direction; namely, the angular momentum of the [Ar'CN-] * complex, J, should be aligned perpendicular to the beam axis.22 Registry No. CN, 2074-87-5; BrCN, 506-68-3; Ar, 7440-37-1. Such a tendency of rotational alignment has been observed for (24) M. N. R. Ashfold, M. T. Macpherson, and J. P. Simons, Top. Curr. reactive scattering processes involving formation of rare-gas halide 86, 1 (1979), and references cited therein. ~ o m p 1 e x e s . lIf~ ~the ~ complex is in a "loose-coupling" ~ t a t e , ' ~ . ~ ~Chem., (25) J. M. Hollas and T. A. Sutherley, Mol. Phys., 22, 213 (1971).

+

+

(22) In the following discussion, j refers to the an ular momentum vector of the products CN(B), J to that of either the [Ar9CN-]* complex or the [BrCN]* precursor, and I to the orbital angular momentum vector of the product CN(B) and the separating atom. (23) D. S. Y . Hsu and D. R. Herschbach, Faraday Discuss. Chem. Soc., 55, 116 (1973).

+

(26) J. Derouard, T. D. Nguyen, and J. Sadeghi, J . Chem. Phys., 72,6698 (1980). (27) A. C. Vikis and D. J. LeRoy, Chem. Phys. Lett., 22, 587 (1973). (28) Y . Ozaki, T. Kondow, and K. Kuchitsu, Chem. Phys., 77,223 (1983). (29) Y. Fukuda, K. Suzuki, T. Kondow, and K. Kuchitsu, Chem. Phys., 87, 389 (1984). (30) Y. Fukuda, T. Kondow, K. Kuchitsu, K. Tabayashi, S . Ohshima, and K. Shobatake, to be submitted for publication.