4712
J . Phys. Chem. 1991, 95, 4712-4718
EELS
volution of the valence and conduction-band states of the material investigated and should be closely related to the energy-loss features observed for the XPS peaks. In this case, the kinetic energy of the primary electrons is 206 eV instead of 1200 eV. The rise in intensity of the CmEELS signal occurs a t a larger energy loss than for the HOPG and with a somewhat smaller initial slope. In addition, the intensity of the loss signal from Cbogoes to zero, whereas there is a nonzero minimum loss intensity for the HOPG spectrum. These facts are indications of a fully open bandgap for the Cbo,as compared to HOPG, which has a relatively large bandgap except along one line in the Brillouin zone.” The energies of the plasmon loss features in the EELS spectra are consistent with the loss peaks observed for the C 1s XPS data.
Eo = 206 eV
L
56.0
50.4
.. .
4.
a.
.
*
,
44.8
I
392
,
I
33.6
/
1
28.0
,
c
22.4
,
’
,
16.8
’
.
J
111
.
-
5.6
l
-0.0
Energy Loss (eV) Figure 6. The EELS spectra, presented in integral mode, for Cm and H O E . The primary electron beam energy was 206 eV, and the heights
of the elastic peaks have been normalized to each other. atoms. The fact that the Cm Auger spectrum resembles that of amorphous carbon indicates that the hybridization of the carbon atoms in Cbois not strictly sp2 but that the curvature of the molecular surface introduces some sd character into the molecular orbitals. The EELS spectra for the two allotropes are shown in Figure 6. In the few-electronvolt regime, EELS is essentially a con-
Summary Thin films of solid Cbohave been grown on Si( 111) substrates by sublimation in UHV. The films deposited from an unpurified source resulted in epitaxial layers consisting of close-packed planes of molecules stacked on top of one another, but the coherence length for X-ray scattering was less than 200 A. The electron spectroscopic data of these films show that the hybridization of the carbon is primarily sp2, but with a significant admixture of sp3 hybridization. The photoemission line shape and the EELS data reveal a fully open bandgap in solid Cbo,which is consistent with preliminary conductivity measurements. Acknowledgment. This work was supported in part by the Office of Naval Research under Grant “14-90-5-1178.
Quenching of NH(alA,vN= 1,2) after HN3 KtF-Laser Photolysis W.Hack* and Tb.Mill MPI f i r Strbmungsforschung,Bunsenstrasse 10, 0-3400 Gbttingen. FRG (Receiued: March 5, 1990; In Final Form: January 3, 1991)
-
-
The photolysis of HN,(%), HN3(%’A’) + hu NH(a’A,u’? + N2(X1Z;), at h = 248 nm was studied in a quasistatic laser photolysis LIF cell. NH(a’A) with u ” = 0-3 was detected directly by the c(’II,u’=O) a(’A,u’? excitation spectra. The transitions c(’II,u’=l) a(’A,u”=&3) were also detected. The initial concentrations of NH(a1A,u”=&3) after single-photon photolysis were determined as [NH(a,u”=O)]/[NH(a)] = 0.32, [NH(a,u”= I)]/[NH(a)] = 0.47, [NH(a,u”=2)]/[NH(a)] = 0.17, and [NH(a,u”=3)]/[NH(a)] = 0.038. The deactivation rate constant for
-
NH(a,u”=l)
+ N2
-
prod.
was found to be kl = (1.7 f 0.2) X IO” cm3/(mol s) and the value k2 = (1.7 f 0.2) for the rate constant of the reaction NH(a,u”=2) + N2 prod.
(1) X
IO1’cm3/(mol s) was measured (2)
The equivalent quenching rates for HN3, instead of N2, were determined to be k3 = (7.6 f 0.8) X I O l 3 cm3/(mol s) and k4 = (7.0 f 0.8) X IOl3 cm3/(mol s), mpectively. For He as a quenching gas, deactivation rates k = (5.2 1) X 108 cm3/(mol s) were obtained for u” = 1 and k = (6.2 & 1) X IO8 cm3/(mol s) for u” = 2.
Introduction
-
The photolysis of HN, HN,(%)
+ hu
NH(a’A,u’’,J’?
+ N2(X’Zg+)
is a frequently used NH(a) source, which has been studied for more than 60 years.’ The overall decomposition products were OO22-3654/91/2095-47 12$02.50/0
analyzed by Thrush.2 Several authors, in particular McDonald and co-workers have studied the state-specific steps in the photolytic dec~mposition.~-’The laser induced fluorescence (LIF) (1) Beckman, A. 0.;Dickinson, R.G.J . Am. Chrm. Soc. 1928,50, 1870; 1930,52, 124, (2) Thrush, B. A. Proc. Roy. Soc. London, A: 1956, 235, 143.
0 1991 American Chemical Society
Quenching of N H after HN3 Laser Photolysis technique was applied to detect the product NH(a’A,u”=O,J”= 2-1 2) via the N H c(’ll,u’=O)a(’A,u”=O) tran~ition.~.~.~.*-” The detection of vibrationally excited imidogen NH(a,u’= 1) was attempted. The ratio of the vibrationally excited to ground state concentrations was determined to be [NH(a,u”= I)l0/ [ N H ( ~ , U ” = O )I ] ~0.051 .3 This observation is in contradiction to the results obtained also with LIF (NH c-a) by Rohrer’O and by Stuhl et a1.,I2who detected considerable amounts of NH(a,0”-1). The authors observed a ratio [NH(a,u”=l)],/[NH(a,u”=O)l0 = 0.72, taking into account the predissociation, which, to a different extent, takes place in the states of NH(c), significantly lowering the fluorescence quantum yield. The rotational temperatures of the two vibronic states were determined to be T,(u”=O) = 1160 K and T,(u”=l) = 1155 K. NH(a) in vibrational states (0’22) could not be found by Stuhl and coworker~.’~*~~ When the HN3 photolysis was used as an NH(a) source to study the NH(a) quenching by N2, the formation of NH(a,u”=O) was retarded14due to vibrational relaxation in the NH(a) vibrationally excited states. This can be considered as an indirect detection of NH(a,u’30), since the formation of NH(ap’30) in the HN3 photolysis at XL = 248 nm has to be assumed in taking into account the results of this work, whereas at XL = 308 nm no indication for a formation of NH(ap’30) was 0b~erved.I~From the deviation of the linear quenching plots (In NH(a) vs t ) at short reaction times, the ratio [NH(a,u”21)]/[NH(a,u”=O)]o can be given quantitatively. The kinetics of NH(a) was, up to now, studied mainly for the vibrational ground state. For NH(a,u”=O) the rate of the depletion NH(a,u”=O)
+M
-
NH(X)
+M
has been measured for several quenching gases (M).I2-l5 The quenching of an initially excited vibronic state (NH(a,ur)) NH(a,Q’?
+M
-
NH(a,q’?
+M
has not yet been studied directly, however an estimation for the NH(a,u”=l) rate of depletion by N2 is given in ref 10. The object of this work was to study the primary vibrational states of NH(a) by HN3 laser photolysis. The excitations NH(c’ll,u”=O) NH(a1A,u”=&3) and an excitation spectrum of NH(aIA,u”=l) were applied to the transition NH(c’ll,u’=l) prove the presence of NH(a,u”=l) as well as NH(a,u”=2 and 3) directly. The kinetics of NH(a,u”=&2) were studied to prove that the formation mechanism of NH(a,u”=O) reported in ref 14 is due to the existence of NH(a,u”?l), to confirm the quantitative ideas about the channel distribution in the HN3 photolysis, and to determine the rates of the vibrational deactivation of NH(a,u”=1,2) with N2 and the precursor molecule HN3.
-
-
Experimental Arrangement The experimental arrangement consisting of a quasistatic exciplex laser photolysis cell and a laser induced fluorescence (LIF) detection device has been described in detail el~ewere,‘~The NH(a) radicals were produced via photolysis of HN3(X) at XL (3) Dekoven, B. M.; Baronavski, A. P. Chem. Phys. Len. 1982,86, 392. (4) McDonald, J. R.; Miller, R. G.; Baronavski, A. P. Chem. Phys. 1978. 30. 133. (5) Baronavski, A. P.; Miller, R. G.: McDonald, J. R. Chem. Phys. 1978, 30, 119. (6) Bower, R. D.; Jacoby, M. T.; Blauer, J. A. J. Chem. Phys. 1987,86, 1954. (7) McDonald, J. R.; Miller, R. G.; Baronavski, A. P. Chcm. Phys. L r r r . 1971, 51, 57. (8) Piper, L. G.; Krech. R. H.; Taylor, R. L. J. Chrm. Phys. 1980,73,791. (9) Kenner, R. D.; Rohrer, F.; Stuhl, F. J . Chcm. Phys. 1987.86, 2036. (IO) Rohrer, F. Ph.D. Dissertation, Bochum, Germany, 1987. ( I 1 ) Cox, J. W.; Nelson, H. H.;McDonald, J. R. Chcm. Phys. 1985, 96, 175. (12) Rohrer, F.; Stuhl, F. J. Chrm. Phys. 1988, 88, 4788. (13) Freitag, F.; Rohrer, F.; Stuhl, F. J . Phys. Chrm. 1989, 93, 3170. (14) Hack, W.; Wilms, A. J . Phys. Chem. 1989, 93, 3540. (15) Rohrer, F.; Stuhl, F. Chcm. Phys. krr. 1984, 1 1 1 , 234.
The Journal of Physical Chemistry, Vol. 95, No.12, 1991 4713 TABLE I: Experimental Details on Applied Fluence 8 d !%tuntion Effects in the NH M Tnmitioas significant dev from linearity 6 appl for of IF(6)at @, meas of re1 transition fluence range c(u’ka(u’3 6.mJ/cm2 uJ/cm2 DODS.. uJ/cm2 >30 21 0-0 0.012-1.8 0-1 1-1 1-2 1-3
0.13-2.8 0.1-39.6 0.048-56.0 1.5-24.3
>700 >300 >500 >6000
170 94 92, 74 600
= 248 nm with an KrF exciplex laser. Two different laser devices where used. For the N H c(IlI,u’=l) a(lA,u”=l) transition the LIF excitation laser was an exciplex-pumped dye laser (FL 3002 Lambda Physik) using the BM-terphenyl dye. The photolysis laser was an exciplex laser (Model LPX 205iCC Lambda Physik) with a pulse energy in the cell of about 150 mJ and a pulse duration of q / 2 = 15 ns, resulting in a fluence of about 400 mJ/cm2. In some experiments, the dye laser was pumped with the same exciplex laser that was used for the HN3 photolysis. A beam splitter divided the exciplex laser beam so that approximately 10% of the photolysis laser pulse energy was directed into the dye laser; the pulse energies were about 1.5 mJ, with a cross section of 3 mm2. The fluence of dye laser radiation in the cell was about 50 mJ/cm2. The LIF pulse was delayed with respect to the photolysis pulse by about 10 ns. In most experiments two exciplex lasers were used. The dye laser was pumped with the XeCl exciplex laser (LPX205). and another, KrF exciplex laser (LPX205, Lambda Physik) was used for photolysis. The fluence of the photolysis laser was varied in the range 120 I$/mJ cm-* I600. In most experiments a fluence of 180 mJ/cm2 was applied. The photolysis pulse had a duration of T l / 2 = 15 ns. The intensities of the dye laser, which were measured with a pyroelectric energy detector (Scientech, Scientific Instruments) are listed in Table I. The N H c(’II,u’=O) a(IA,u”=l), N H c(’II,u’=O) a(’A,u”=2), and NH c(’lI,u’=O) a(lA,u’’=3) transitions were generated by pumping the dyes DMQ, DPS and Coumarin 47, respectively. In the wavelength range 324 IX/nm I343, pterphenyl was used to pump the 0-0 and 1-1 transitions. The dye laser was calibrated with the lines of a Xe low-pressure lamp (Ultra-Violet Products) at A = 462.4275 nm and X = 467.1225 nm and a Hg low-pressure lamp at X = 404.656 nm and 407.783 nm by using a monochromator (Jobin Yvon THR f/13.6). The spectra were obtained by observing the emission from NH(clII,u’=O) to NH(a’A,u”=O) with the aid of an interference filter (maximum transmission T, = 19% at X = 326 nm, fwhm 3 nm, Schott) . The spectra were recorded in both cases via a photomultiplier (98 17QB, EMI) and a transient digitizer (Tektronix 7912/AD). The gas flow in the cell was in the range (0.04 Iii/m s-’ I 0.5) and in all experiments was so rapid that no photolysis products could accumulate to significant concentrations. The concentration ratio [NH(a’A)]/[HN3] was on the order of estimated by using the cross sections determined by McDonald et a1.,I6 thus, the reactions of NH(alA) with itself were considered to be of minor importance. Gases of the highest purity commercially available were used without further purification. HN3 was produced and purified as described in ref 14.
-
+
Results and Discussion In the photolysis of HN3(g) HN3(R) + hu (248 nm)
-
-
-
NH(a,u’?
+ N2(X)
the energy is sufficient to excite NH(a) to the vibrational state u” = 8. The gas-phase laser-photolysis of HN3 was studied at
low pressures in a quasistatic LIF cell. Both the primary products (16) McDonald, J. R.; Rabalais, J. W.; McGlynn, S.P. J . Chem. Phys. 1970, 52, 1332.
4714 The Journal of Physical Chemistry, Vol. 95, No. 12, I991 410”’
010-11
1-PIO-1) 13
17
I2 ‘I
16 15 1L 13 11 I’ 10 I’ 9 ” 8 ‘I
12 7’
11 10 9 8 7 6 5 1 3’‘2 6’ 5 ‘L I 3’ 2
t “b
Hack and Mill
PIO-21
RIO-11
1 I I Ill
O5
I
n m
n m
1
n Loo
27 604
.
-
Bvoc Icm’‘ 1 1
1
1
372
370
368
366
AA,r lnml
1
I
364
362
-
I I L
the NH(c’II) state. and the kinetics of these products were investigated. The results are therefore presented in two sections; first, the excitation spectra are described and discussed, and second, results on the elementary reactions of NH(a,u”) with HN,, N,, and He are given. Excitation Spectra for NH(c,u‘=O) .-NH(a,u”). NH(a,u”4 - 3 ) was detected after excitations to the NH(c’II) state via the transitions NH(clII, V‘ = 0, J’)
---
I
24500
cm-ll
C,,
1. Excitation spectrum of NH(c’II,u’=O) NH(aIA,u”=l). The observed splitting of the lines with high P i s a result of A doubling in F i i
I
24LOO I
1
1
L10
LO9
LO8
lnml
A,,,
Figure 2. Excitation spectrum of NH(clII,u’=O) assignment. IF
(0
uI
-
NH(a’A,u”=2) with
-
10
10
P lc.0 b 0) P (c.0-0.31
3
4
15
1b
(1 l c . 0
0 Ic.0
-
b.0)
- 0.31
NH(a’A, V” = 0, J”) 324 I lJnm I 343 NH(a’A, v” = 1, 1”) 361 I lJnm I372 NH(a’A, v” = 2, J”) 407 I lJnm I 41 1 NH(a’A, v“ = 3,J”) 462 I lJnm I 465
and in the case of NH(alA,u”= 1) additionally via the transition NH(c’II,u’=l,J? .-(a’A,u”=I,J”j
336 I X/nm I 344
-
The excitation spectrum of NH(alA,u”= l,J”),which results from the NH(c’II,u’=O) NH(a’P,u”=l) transition is shown in Figure 1 . The emission was recorded a t a total pressure of p = 2.4 mbar. The gas consisted of He (p(He) = 1.8 mbar) and H N 3 (p(HN3) = 0.6 mbar). The delay between photolysis and dye laser pulse was A1 = 200 ns. The spectrum shows lines of the P, Q, and R branches (Figure 1). For larger values of J the A doubling was clearly observed in the spectrum. The A doubling is mainly due to the splitting in the c l n state, whereas the splitting in the alA state is below experimental res~lution.’~The spectrum appears immediately after photolysis (minimum delay time of 2 ns), indicating that NH(a,u”=l) is a primary product of the HN, photolysis at X = 248 nm. The intensity of this transition was compared to that of the NH(c’n,u’=O) NH(a’A,u”=O) spectrum (324 I X/nm I343), in order to determine the relative populations of the NH(a,u”=O,I) states. From this comparison nearly equal populations of the two states can be deduced by using electronic-vibrational transition probabilities measured in this work. The results obtained here are in reasonable agreement with the results published by Stuhl et al.? who used the transition probabilities measured by Lents.Is The excitation spectrum of NH(c’II,u’=O) .-(alA,u”=2) is shown in Figure 2. The emission was recorded under the same experimental conditions as given above for the 0-1 transition. The spectrum consisted of the P and Q branches. In the equation
-
E,,, = B(J(J
+ 1) - A’) - D(J(J + 1) - A’)’ + H ( J ( J + 1) - A’)3
the rotational constant B(NH(a,u”=2)) = 15.203 f 0.002 cm-l and the centrifugal distortion constants D(NH(a,u”=2)) = 1.61 (17) Ubachs, W.; Meyer, G.; Ter Meulen, J. J.; Dymanus, A. J . Mol. Spectrosc. 1986, I I S , 88. (18) Lents, J. M. J. Quant. Spectrosc. Radiat. Transfer 1973, 13, 297.
I
21500
21550
21m
21650-
Icm-’]
ijMc I
I
L65
164
A,,
Inml
I
I
(63
L62
-
-
Fmre 3. Excitation spectrum of NH(c’II,u’=O) NH(a1A,u”=3)with assignment. Superimposed are some lines from the NH(c’II,u’=O) NH(b’Z+,u”=O)spectrum, obtained under the experimental conditions used (see text). f 0.03 X cm-’ and H(a,u”=2) = 0.9 f 0.7X IC7cm-’ were determined from the spectrum.lg The origin of the transition NH(clII,u’=O) NH(a’A,u”=2) is ijO2= 24484.61 cm-’.19 The excitation spectrum for the N H transition c(’ll,u’=O) a(’A,u”=3) is shown in Figure 3. The emission was recorded at p = 10.7 mbar (p(He) = 9.1 mbar,p(HN3) = 1.6mbar). The delay between photolysis and analysis pulses was At = 200 ns. This spectrum is assigned to the P and Q branches of this transition. By calibrating the dye laser with the Xe lines, the origin of the 0 .-3 band can be calculated to be at ZO3= 21 591.60 The spectrum can be assigned by using the constants of the NH(alA,u”=3) state: B(NH(a,u”=3)) = 14.599 f 0.003 cm-I and D(NH(a,u”=3)) = 1.56 f 0.06 X cm-’.I9 Superimposed on the c a spectrum are some lines of the NH(bZ+,o”=O) state, which was excited to NH(c’n,v’=O) by the dye laser. NH(b) was found to be a secondary product of the HN, photolysis under the experimental conditions applied, since it was not formed directly after the photolysis pulse but reached maximum concentration a t At = 1 ps. Besides the NH(a’A) vibrational states, the only other detected N H vibronic state formed directly in the KrF photolysis of HN3 was NH(A3n,u=O), as already reported in ref 10. Excitation Spectrum for NH(c,u’=l) NH(a,u”=l). The excitation spectrum of NH(aIA,u’’= I ) , which results from lines of the P, Q, and R branches of the NH(c’II,u’=l) NH-
-
-
-
+
(19) Hack, W.; Mill, Th. J . Mol. Spectrosc. 1990, I l l , 358.
-
The Journal of Physical Chemistry, Vol. 95, No. 12, 1991 4715
Quenching of N H after HN3 Laser Photolysis
I, 1a.u.I
P-bmwh
9
88
71
66
5
4
3
Pll-11
2
Tnmll
1-
05-
3 86 5
330 29200
29400
29600
A,,
29800
Jvoclcm’’ I 3L2
340
*
370
360
[nml
1 338
A,,, lnml
Figure 4. Excitation spectrum of NH(c’ll,u’=l)
336
-
-
Ic1n.v’x 1 l-lalA,v”=Ol
NH(alA,u”=l) with assignment of P, Q,and R branches and the transition u’ = 0 u” = 0 with part of the P and Q branches. For long wavelengths the signalto-noise ratio was improved.
( c l n , v ’ = l l-lalA,v”zll
lclTt,v’.l
Hbl$v’:Ol
-
(alA,u”=l) transition is shown in Figure 4. Superimposed are some lines of the Q and P branches of the transition (c,u’=O) (a,u”=O). The spectrum was recorded at a total pressure of p = 2.0 mbar; the partial pressure of He was 1.5 mbar and for HN, the partial pressure was 0.5 mbar. The delay between photolysis and analysis pulses was At = 10 ns. The first 30 ns of the emission were observed with the digitizer. The intensity and thus the signal-to-noise ratio obtained in the u’=l u”=l excitation spectrum was smaller by about 2 orders of magnitude, under the experimental conditions applied, than the intensity and/or the signal-to-noise ratio of the u’=O u”=1 excitation spectrum, which might explain why it was not observed earlier? The relative populations of the NH(a,u”=0,1) states can not be obtained from the lines of the 0-0 and 1-1 transitions, respectively, without taking into account the spectral sensitivity of the detection system, since the fluorescence is observed from different excited states. The spectral sensitivity was measured and used for the correction of the spectral intensities. The N H transition c(u’=l) a(u”=l), which is not favored by the smaller fluorescence quantum yield of the c*II,u’=l state with respect to the u’=O state, can be useful, since it offers a convenient detection method for NH(a’A,u”= 1) in the spectral vicinity of the N H A(311,u=O) X(3Z-,u”=O) and c(’II,u’=O) a(IA,u”=O) bands. For kinetic studies in which NH(X), NH(a,u”=O), and NH(a,u”=l) have to be detected in the same experiment, this, c(u’=I) a(u”=l), can be convenient. Population of the NH(a,u”) States after the HN3 Photolysis. The population of NH(a,u”=l-3) over the sum of all states with u” 1 0; Le., the detailed branching ratio in the HN, photolysis
-
-
305
-
310 “ 3 3 5
L15
3LO
A,
L20
L25
I nml
Figure 5. (a, top) Fluorescence spectrum of emission of NH(c,u’=O). The signals are corrected due to the spectral sensitivity of the detection system. (b, bottom) Fluorescence spectrum of emission of NH(c,u’=l) NH(a,u”=O-3). The signals are corrected as stated in part a.
-
t
-
150
1
IF
/
/’
+
-
+
HN&
!% NH(a’A,v”=O) + N2@) NH(a’A, v” = 1)
+ N2(X)
NH(a’A, v” = 2) + N2(X) NH(a’A, v” = 3)20 + N2(X) can be determined from the relative intensities of the spectra recorded at identical HN3 pressures for the transitions NH(clll,u’=O) NH(a’A,u”=O,l) and NH(clII,u’=l) NH(alA.u”= 1-3) in this work, with relative Einstein transition probabilities of spontaneous emission (AJJt),which were determined from the fluorescence spectra as shown in Figure 5a,b. The emission intensities for the N H transitions c(u’=O) a(u”=O,l) are shown in Figure 5a and for the transitions c(u’=l) a(u” 4 - 3 ) are shown in Figure 5b. (The transition c(u’=l) a(u”=O)
-
--
(201 During the revision of this D a m also NH(a.u”-4) was observed: the amount can Kowevet be assumed’td be small compared to NH(a,o”k3).
200
100
Ep,lmJ
300
1
Figure 6. LIF intensity IF(NH(a,u”=3)) versus energy of the photolysis
laser.
was not used in this work.) Thus, the relative Einstein transition probabilities of absorption (BJJ.)are calculated for these transitions, which are given in Figure 5a,b. In all experiments the P(2) rotational transition was excited. The relative populations were determined from the fluorescence intensity exciting the transition NH(c,u’=O) NH(a,u”=O,I) and NH(c,u’-I) NH(a,u”=l-3). The transition probabilities described above were corrected with a term P(J”)/ZytP(J”), in which P(J’9 is the population of J”at rotational equilibrium a t room temperature. The dve laser intensity was reduced as given in Table I to avoid saturition effects. Tge photolysis IaserTntensity was also varied
-
+
Hack and Mill
4716 The Journal of Physical Chemistry, Vol. 95, No. 12, 1991 TABLE 11: Relative PoDuI.tio11~of NH(~a,r”l vib re1 pop.‘ re1 pop.b state u NH(a,u‘?/&pNH(a,u’? NH(a,u”l/&NH(a.u’? 0 1 2 3
0.32 0.47 0.17 0.038
/*’
0.26 0.28 0.22 0.24
‘This work. bReference 20. b
Figure 8. First-order rate constant k’ vs [HN,] for depletion of NH(a,uf’=1,2) by HN3: ( 0 )u” = 1; ( 0 )u” = 2.
t
7.0 1
0
5
10
15
25
20
30 Ips1
At
Figure 7. Fluorescence intensity of NH(a,u”= 1) as a function of delay time for different HN3 concentrations: (*) p(HNJ = 0.0012 pbar, p(He) = 18.7 mbar; ( 0 )p(HN3) = 2.4 pbar; (0)p(HN3) = 4.6 pbar; (A)p(HN,) = 6.7 pbar; (m) p(HN,) = 8.6 pbar.
to prove that no multiphoton effects are responsible for the population of the higher vibrational states. A linear dependence of IF(NH(a,u”=3)) versus the energy of the photolysis laser as shown in Figure 6 indicates that NH(a,u”=3) is produced in a single-photon photodissociation. Since for the highest vibrational state (u”=3) no indications were found that it is formed in a multiphoton process, for the lower vibrational states also a single-photon photodissociation can be assumed. The relative population of an NH(a,u”) state after the single-photon photolysis of HN3(fC) at X = 248 nm can be given as stated in Table 11. The nascent NH(a’A) vibrational distribution from 266-nm photolysis of HN, was determined recently by Nelson and McDonald.21 The relative populations are also listed in Table 11. For the lower vibrational state they also obtain an inversion, but for the high vibrational states the relative populations found in this work are significantly lower. Elemenrary Reactions of NH(a,u”). The kinetics of NH(a,u”=O) also provides indirect evidence of the formation of higher states of NH(a) in the HN, photolysis: HN,
+ hv
(248m) 4 N H ( B , v ” = O ) + Nz(X) -C NH(& V” 2 1) + NZW)
In addition, quantitative information on the channel distribution in the photolysis reaction is obtained from the kinetics of NH(a,u”21). since it has to be proved that the observed population is not different from the initial distribution due to collision-induced vibrational relaxation. Moreover the kinetics of NH(a,u’? is itself of interest. The kinetics of NH(a’A) in its (o”=l) and (u”-2) states was investigated for the quenchers HN,, He, and N2. For the depletion of the vibrationally excited states by HN, NH(a,u”=l) + HN, prod. (3)
-
the NH(a,u”=l) depletion was measured for different HN, concentrations. The fluorescence intensity as a function of the delay time At between the photolysis and probe laser pulses is shown as a In fF(NH,a) vs At plot in Figure 7. Under all ex~
(21) Nelson, H. H.; McDonald, J.
lication.
R.J . Chcm. Phys., submitted for pub-
t
0
I
I
1
20
LO
60
100 [PSI
80
At
Figure 9. Semilogarithmicplot of [NH(a,u”=2)] as a function of delay
time for different HN3 concentrations: (m) p(HN3) = 2.4 pbar; (0) p(HN3) = 7.4 pbar; (A)p(HN3) = 11.2 pbar; (0)p(HN3) = 17.0 pbar. TABLE III: Experimental Results for Qwnchhg of NH(a,r”=1,2) by
HNI vib state u“
p(HN3). pbar
1
2.4 4.6 6.7 8.6
10.10 10.08 10.08 10.05
2-30 2-32 2-26 2-20
0.93 1.69 2.38 3 .00
2
2.4 7.4 11.2 17.0 17.0 46.5 69.2
18.63 10.03 10.12 10.08 10.08 10.01 10.02
3-60 3-100 2-20 2-20 2-18 2-35 2-2s
0.53 2.42 3.53 5.39 5.34 12.9 17.7
..
perimental conditions straight lines were observed. The observed reaction times were in the range 1 IAt/ps I26. The first-order rate constants determined from the slopes of the plots (Figure 7) as a function of [HN,] are shown in Figure 8. The slope of that plot yielded an overall rate constant of
k3 = (7.6 f 0.8) X IO1, cm3/(mol s) which includes vibrational and/or electronic quenching and chemical reaction. The stated errors are 3u excluding systematic errors. For NH(a,u”=2) HN, prod. (4)
+
-
the experimental details are summarized in Table 111. Some plots of In IF(NH,a) vs At are shown in Figure 9. For long reaction times the slope becomes smaller as seen in Figure 9. This effect, which is more pronounced a t higher HN, concentration, can be interpreted as a reaction of a species produced in the H N 3 pho-
The Journal of Physical Chemistry, Vol. 95, No. 12, 1991 4717
Quenching of N H after HN, Laser Photolysis
TABLE I V Experimental Results for Quenching of NH(O,r’’=l) by
In INH(a.v”=1, J = 511
N. ~~
I
p(He), mbar
p(HNA mbar
2.43 8.24 14.49 15.20
0.01 0.04 0.0 1 0.03
0.001 0.004 0.001 0.003
t
.*\ L -
P(N~), mbar
In [NH(a.v”= 2 . J
k’, IO‘ s-l
At, ps 6-80 2-30
1.84 5.73 8.57 9.62
2-15 2-20
= 511
c
3 0
5
10
15
20
T
At [psl Figure 11. Semilogarithmic plot of [NH(a,u”=2)] as a function of reaction time: p(Nz) = 19.98 mbar, p(HN,) = 11.3 pbar.
/
10
15
XI
25
20
/
*
PN2Im bar 1
Figure 12. First-order rate constant k’ vs [N2]for depletion of NH(a,u”=2) by N1.
0’31) is generated in the KrF photolysis of HN3 in the amount given above. Assuming no formation of NH(a,u”=l) by vibrational deactivation of NH(a,u’3 I), the NH(a,u”=l) depletion can be described by the rate constant k l = 1.3 X lo1’ cm3/(mol s). This value was obtained directly from the slope of the In INH(a,J,II) vs At plots, as shown for example in Figure 10. All data for k1are summarized in Table IV. For the interpretation of the value of kl, it has to be taken into account that formation of NH(a,u”= 1) by vibrational relaxation of NH(a,u’3 1) would retard the decay of NH(a,u”=l). The correction due to this effect was based on the determined relative concentrations of the vibrational states of NH(a’A), as given above, whereby vibrational relaxation of NH(a,u”=2-4) was considered. For the vibrational deactivation NH(a,u’? + N2 NH(a,u”-I) N2
-
+
a rate constant of k = 1.2 X l o l l cm3/(mol s) was estimated from data in the l i t e r a t ~ r e , ’ ~and - l ~for the electronic deactivation NH(a,u’? + N2 NH(X) + N2
-
(22)
Hack, W.; Wilms, A.
Z. Phys. Chcm. (Munich) 1989. 161, 107.
the rate for NH(a,u”=O) of k = 0.5
X
10” cm3/(mol s) was
J. Phys. Chem. 1991, 95, 4718-4722
4718
The overall rate constant k2 = 1.5 X 10” cm3/(mol s) can be evaluated from the plot k1vs p(N2), as shown in Figure 12. This leads to the value k2 = (1.7k 0.2) X 10” cm3/(mol s)
0
20
A t [PSI
4o
Figure 13. Semilogarithmic plot of NH(a,u”=O) concentration proille following photolysis of HN3 at XL = 248 nm in the presence of N2, as given in ref 14: p(HN3)= 1.7 pbar, p(N2) = 21 mbar.
assumed for all u”. Inserting these data in a computer simulation allows a corrected rate constant
k l = (1.7f 0.2)
X
10” cm3/(mol s)
to be obtained. This value is slightly lower if a two-quanta deactivation mechanism NH(a,u”=2) N 2 NH(a,u”=O) N2
+
-
+
is taken into consideration. The quenching of NH(a,u”=2) by N2 NH(a,u”=2) + N 2 prod.
-
if we apply the same corrections for vibrational deactivation of higher states as described above. In the quenching experiments of NH(a,u”=O) with N2,I4 in which the NH(a) was produced from the HN, photolysis a t XL = 248 nm, an increase was found in the NH(a,u”=O) profile for small reaction times (Figure 13). For probe laser delay times At > 22 pj a linear decrease of In IdAt), i.e. of [NH(a,u”=O)](At), was observed. The straight line in Figure 13 is an extrapolation of the measured NH(a,u”=O) concentration toward At = 0. The deviation from the linear plot in Figure 13 can be understood as a formation of the rovibronic state NH(a,u”=O,J”) by a source different from the direct HN, photolysis. Rotational relaxation can be ignored, since it is completed within a significantly shorter time scale (ns). The reproduction of NH(a,u”=O) can be explained now by vibrational relaxation. The quenching rate constants of NH(a,u’? by N2 are all significantly smaller than gas kinetic collision numbers; thus, an activation barrier can be expected, which was determined directly for N H ( ~ , U ” = Oand ) ~ ~also recently for NH(a,u”= 1)24 when the temperature dependence of the collisional quenching of NH(a) was measured. Acknowledgment. We are grateful for the support and encouragement given to us by Prof. H. Gg. Wagner. Financial support of the DFG, SFB 93 is acknowledged.
(2)
is shown in Figure 1 1 as a In[NH(a,u”=2)] vs At plot. Plots of this kind were linear under all observed experimental conditions.
(23) Nelson, H. H.; McDonald, J. R.; Alexander, M. A. J . Phys. Chem. 1990, 94, 3291. (24) Hack, W . ;Rathmann, K. J. Phys. Chem., submitted for publication.
The Reaction of Superoxide Radical Anion with Dithiothreitoi: A Chain Process Nan Zhang, Heinz-Peter Schuchmann, and Clemens von Sonntag* Max- Planck-Institut fur Strahlenchemie, Stiftstrasse 34-36, 0-4330 MiilheimlRuhr 1. Germany (Received: July 24, 1990; In Final Form: January 31, 1991)
The radical-induced oxidation of dithiothreitol (DTT) by O2to 4,5-dihydroxy-l,Zdithiane (ox-DTT) in aqueous solutions of pH 8.7proceeds via a chain reaction. Radicals such as OH generated by the radiolysis of N20/02-saturatedaqueous solutions abstract an H atom from DTT. The resulting DTT(-H)’ radicals cyclize and deprotonate, yielding the disulfide radical anion o x - D T - . The latter reacts with O2( k = 7.1 X lo* dm3 mol-l s-l)giving rise to ox-DTT and 02*-.The G value of ox-DTT formation increases with decreasing dose rate, displaying a linear yield vs (dose rate)-II2 relationship indicative of a chain reaction. The chain is inhibited by superoxide dismutase, showing that 02*is a chain carrier. Hydrogen peroxide is not a chain product, from which it is concluded that 02-does not abstract hydrogen from D’IT but rather forms a short-lived complex with the DTT anion which then predominantly decomposes into an RSO’ radical and an OH- ion, (and to a lesser extent into an OH radical and an RSO- ion). In a second step, the RSO’ radical reacts with DTT (probably the anionic form D T ) , thus regenerating DTT(-H)’. The resulting sulfenic acid eliminates water and yields the chain product ox-DTT. An overall propagation rate constant of 35 dm3 mol-l s-’ has been estimated.
Introduction The superoxide radical anion participates in many chemical systems that undergo autoxidation by the action of 02.In particular in aqueous solution, it is released from many peroxyl radicals.’” Due to its slow self-termination reaction it can build ( I ) von Sonntag, C. The Chemical Basis of Radiation Biology; Taylor and Francis: London, 1987. (2) Des, S.;Mieden, 0.J.; Pan, X.-M.; Repas, M.; Schuchmann, M. N.; Schuchmann, H.-P.; von Sonntag, C.; Zegota, H.In Oxygen Radicals in Biology and Medicine;Simic, M.G., Taylor, K.A., Ward, J. F.,von Sonntag, C., Eds.; Plenum: New York, 1988; Basic Life Sciences Vol. 49, p 55.
up to rather high steady-state concentrations. In fact, self-termination of two 02’-radicals is very slow if it occurs a t all (k I0.35 dm3 mol-’ s-I at pH 13), while 02-terminates readily (kll = 9.7 X IO7 dm3 mol-’ s-I) with its conjugated acid HOz’ (pKa(H02’) = 4.8).4 In the living cell where 02’-is an unavoidable byproduct of it can cause considerable damage (e.g. oxygen ~
~~~~~
~
(3) von Sonntag, C. In Free Radicals, Metals Ions and Biopolymers;
Beaumont, P. C., Deeble, D. J., Parsons, B. J., Ria-Evans, C., Eds.;Richelieu Press: London. 1989: D 73. (4) Bielski, B. H. J:; Cabelli, D. E.; Arudi, R. L.; Ross, A. B. J . Phys. Chem. ReJ Dura 1985, 14, 1041.
0022-3654/91/2095-4718$02.50/0 0 1991 American Chemical Society
