J. Phys. Chem. 1984, 88, 3627-3633 TraegeP7 value for the AHfOo(CsHS+)= 1080 kJ/mol or the more recent value of Bombach et al.'* of 1038 kJ/mol. Both of these limits are shown in Figure 7. The discrepancy between our observed onset and the calculated one can again be explained by the kinetic shift. Our sensitivity for detection of this ion at 13.9 eV is considerably less than the corresponding sensitivity of the CSH6+ion at around 12 e v . If we assume that a minimum rate of lo4 s-l at an energy of 13.9 eV is required for significant signal to be observable, we can calculate the statistically expected dissociation rate by using the activation energy as a variable parameter. The calculation with the correct Eo yields a rate of lo4 s-l at an energy of 13.9 eV. This analysis is based on far less certain ground than was the calculation of the Eo for the C O loss from the phenol ion. The reason for this is that we have no dissociation rates for H loss from CsH6+so that we must use the assumed rate a t only one energy. Secondly, the CSH6+ions are not state selected. In fact, they are formed in a distribution of internal energy states. This distribution is approximately the complement of the KERD which produces the C5H6+ions from phenol. For instance, the dip in the KERD of Figure 6 at low translational energies indicates that few CSH6+ions with the maximum possible internal energy are formed. These considerations were taken into account in estimating an activation energy of 3.2 f 0.1 eV. The heat of formation of C5HS+is of considerable interest.38 There is a series of cyclic ions C,H,+, n = 3-8 ( n # 9, whose (37) F. P. Lossing and J. C. Traeger, J . Am. Chem. Soc., 97, 1579 (1975). (38) R. W. Brill, T. J. Buckley, J. R. Eyler, S. G. Lias, and P. J. Ausloos, presented at the 31st Annual Conference on Mass Spectrometry and Allied Topics, Boston, MA, 1983.
3627
energies and structures are now reasonably well-known. The one exception is the case of the five-membered ring. Until recently, the best value for the AHfo(CsHs+)was one obtained by Lossing and Traeger,37which was based on the measured onset of H atom loss from CSH6+. This is precisely the same reaction which produces the ion in the present study. Our rate calculations show that we can expect a considerable kinetic shift in the measured onset. The experiment of Lossing is also subject to this shift, so that we can consider the Lossing value only as an upper limit. A more recent study by Bombach et a1.18 is based on a PEPICO rate study of the sequential dissociation of the toluene ion: C7H8+ C7H7+ CSH5+.The analysis is somewhat similar to the one carried out in this study and is subject to some of the same sources of error resulting from the problems in dealing with sequential dissociation reactions. Our results do not lead to an improved value for the CSHS+heat of formation, except that we have uncovered at least one source of the discrepancy between the Lossing ang Bombach values. The latter is probably closer to the true one.
-
-+
Acknowledgment. M. L. Fraser-Monteiro and L. FraserMonteiro thank the Council for International Exchange of Scholars for Fulbright Fellowships. We are grateful to J. Ronald Hass of NIEHS, Research Triangle Park, for carrying out the CsH5+precursor ion study. Finally, we thank S. Anderson and J. Durant for sending us the phenol ion frequencies prior to publication. This work was supported by a grant from the Chemistry Division of the National Science Foundation. Registry NO. C6H@H+, 40932-22-7; C&+, 29661-18-5; phenol, 108-95-2.
76563-67-2; CsHs',
ArF (193 nm) Laser Photolysis of HN3, CH3NH2,and N2H,: Formation of Excited NH Radicalst H. K. Haakl and F. Stuhl* Ruhr Universitat, Physikalische Chemie I , 0-4630 Bochum, West Germany (Received: November 8, 1983)
HN,, CH3NH2,and N2H4were photolyzed with an ArF (193 nm) excimer laser. Emissions from excited N H in the (A311) and the (C'II) states and also from CN(B) and CH(A,B,C) were observed in the wavelength range 200-500 nm. The NH fluorescence was investigated in detail and in comparison with the recently observed formation of NH(A311) from ammonia. The formation processes and the rotational population of the imidogen radicals, their lifetimes, and their quenching by the parent molecules were studied.
Introduction Irradiation with light from excimer lasers has recently become a new tool for the study of electronically excited molecules.' In order to generate UV emission from fragments of stable molecules either the absorption of more than one laser photon is essential or the parent molecule must be weakly bound. Fluorescences occurring at wavelengths shorter than the photolyzing excimer laser light definitely indicate the occurrence of multiphoton processes.2 Recently, we have begun a search for vacuum-UV and UV emissions from simple molecules generated by ArF excimer laser photolysis at 193.3 nm.2-5 During these investigations we have observed strong N H (ND) ( A 3 n X32-) fluorescence when photolyzing a r n m ~ n i a .This ~ emission was found to occur in a
-
t Presented in part at the International Symposium on Chemical Kinetics Related to Atmospheric Chemistry, Tsukuba, Japan, June, 1982. *Present address: Fa. Buck, Mozartstr. 2, D-8230 Bad Reichenhall, West Germany.
0022-3654/84/2088-3627$01.50/0
two-photon resonance process. Furthermore, it was shown that the excess energy of this process preferentially appears as rotational excitation of the N H (ND) (A311) fragment. We therefore became interested in the possibility of generating N H emission from different parent molecules such as HN3, CH,NH,, and N2H4. In the photolysis of each of these molecules we again observed N H emission. This paper reports the results of these studies and compares them with our previous work on NH, (ND,). Since the completion of the present experiments, reports on the ArF laser photolysis of CH3NH,6 and of N2H47have been pub(1) W. M. Jackson, J. B. Halpern, and C. S . Lin, Chem. Phys. Lett., 55, 254 (1978). (2) H. K. Haak and F. Stuhl, Chem. Phys. Left.,68, 399 (1979). (3) H. K. Haak and F. Stuhl, J. Photochem., 17,69 (1981). (4) K. Shibuya and F. Stuhl, J . Chem. Phys , 76, 1184 (1982). ( 5 ) H. K. Haak and F. Stuhl, J . Phys. Chem., 88, 2201 (1984). (6) N. Nishi, H. Shinohara, and I. Hanazaki, "The Review of Laser Engineering", Vol. 10, The Laser Society of Japan, 1982, p 394.
0 1984 American Chemical Society
3628 The Journal of Physical Chemistry, Vol. 88, No. 16, 1984
Haak and Stuhl
1
NH(c'll-a'A
32L
I
X31
NH (A311-
I
J
328
332
Wavelength
336
/ nm
Figure 1. Spectrum of the NH(c'II,u'=O+a'A,u''=O) emission in the ArF laser photolysis of 0.13 mbar of HN3. The spectral resolution is 0.2 nm and the a p e h e duration was 0.5 ps. The laser energy was 30 mJ/cm2
lished. In each case, the results we have obtained differ in certain details with those reported by the other authors. For these two parent molecules, we therefore summarize our results pertinent to the formation of excited N H and limit the discussion to the generation process and to possible reasons for the differences with previous work. To our knowledge, no studies of the A r F laser photolysis of hydrazoic acid have been reported and we present these results in more detail.
Experimental Section The apparatus used for the present experiments has been described previously in detail.5,s Briefly, an unfocused ArF excimer laser (193.3 nm = 6.42 eV; typically 50 mJ cm-2) was used to irradiate flowing samples of HN,, CH3NH2, and N2H4. Fluorescence emissions were observed during and after the laser pulse by using short aperture durations of up to several microseconds. Fluorescence spectra were recorded in the wavelength range of 200-500 nm. Decays of the emissions were determined for lifetimes longer than 30 ns. The CH3NH2and N2H4samples had the following stated purities: CH3NH2, 97% (major impurities: (CH3)2NHand (CH3),N; Messer Griesheim); N2H4, 99% (free of water; Carl Roth). The H N 3 samples were prepared according to the method of Pannetier and M a r g i n e a n ~ . ~After the HN, was dried, it was stored at the temperature of C 0 2 ice. Shortly before a run, about 10 torr of H N 3 was filled into a glass bulb of 5000 cm3 for immediate use.
Results HN,. Upon irradiation of HN, with light from the ArF laser, several emissions were observed in the wavelength range of (7) W. G. Hawkins and P. L. Houston, J . Phys. Chem., 86, 704 (1982). (8) K. Shibuya and F. Stuhl, Chem. Phys., 79, 367 (1983). (9) G. Pannetier and F. Margineanu, Bull. SOC.Chim.Fr., 2617 (1972).
TABLE I: Emissions Observed (x) in the ArF Laser Photolysis of HN3, CH3NH2, NzH4, and NH3 CH,transition band X/nm HN3 NH2 NzH4 NH3C
NH(A311-.X3Z-)
(1,O)
304.8 335.8 336.8 315.2
x x
x 'x
Xb
xb
x x
x
x"
xb
x
x
x"
xb
x
304.2 325.1 362.1
x x x
452.4
x
358.2 384.6 385.0 385.7 386.8 388.0 314.7 390.0 431.4
x"
x" x" X"
x" x" X XU
X"
"Also observed by Nishi et al." bAlso observed by Hawkins and Houston.' CReference5. 200-500 nm. These emissions and their electronic transitions are listed in Table I. Comparison of the observed integrated intensities, Z,reveals that, under the present experimental conditions (0.1 3-0.26 mbar), the strongest emission originates from the N H (c a) transition (Z(c-a) = 4Z(A-+X)). The triplet emission is about 20 times weaker than the corresponding emission observed under comparable experimental conditions in the photolysis of NH3.5 For the emission array from the clII state, the intensity ratio Z(c+a)/Z(c-b) = 67 was obtained from corrected total intensities of the u' = 0 u" = 0 bands. For the emissions from
-
-
The Journal of Physical Chemistry, Vol, 88, No. 16, 1984 3629
Formation of Excited N H Radicals
I
I
I
I
I
I
33L
330
I
Wavelength
I
I
338
I
362
/ nm
Figure 2. Spectrum of the NH(A311+X3Z-,aU=O) emissions in the ArF laser photolysis of HN, at the experimental conditions of Figure 1. The spectral resolution is 0.08 nm.
io3
IO2
-
NHlc’ll -9
?
z
IO’
-
ICn \ H
IO0
0
b
\ 2
- 10
I
-
-
14
-
10
N H it'll)
4
6
Energy /IO00 Figure 3. Dependence of Nr,,(N9/(2N’+1)
-
a cm-’
10
12
I / s w v 4 on the rotational energy for the triplet and the singlet NH emissions. This data were calculated from the spectra of Figure 1 and 2. The intensities of only the P branches were evaluated.
-
the c a transition, the intensity ratio Z(O,O)/I(O,l) = 30 was observed. When the results of a previous determination of these transition probabilities are used,1° the ratios are predicted to be 31 and 22, respectively. Spectra of the NH(c1n,u”~a1A,u’’=O) and of the NH(A3IJ.,u’=O-X32-,u”=0) emissions are shown in Figures 1 and 2, respectively. It should be noted that the intensity scale for each figure is different. Excitation of rotational levels up to N’ = 11 for clII, v’ = 0 and up to N’ = 27 for A311, v’ = 0 can be observed in these spectra. The intensities, Z, of the rotational lines of the P branches were used to determine the relative rotational population, Nr,,(N?
s,,
The Honl-London factors, required for this calculation were obtained as described before for the tripletS and for the singlet” ( N ’ = J’) emission. Figure 3 displays z/sN‘v4 as a function of energy for both emission systems. The analyses of these plots result in very different rotational temperatures, Trot,of 700 and 4300 K for NH(c’II,u’=O) and (A311,u’=O), respectively. Decays of both singlet and triplet fluorescence were measured for various pressures of HN3. For the singlet system, the decays (10) J. M. Lents, J . Quant. Spectrosc. Radial. Transfer, 13, 297 (1973). (1 1) G. Herzberg, “Molecular Spectra and Molecular Structure, I. Spectra of Diatomic Molecules”, Van Nostrand, New York, 1950.
1,6 0 4
0
0 8
12
Pressure / mbar Figure 4. Decay rates of the fluorescence from the NH(c’II) and NH(A311) states as a function of pressure of HN, and CH,NH2, respectively.
The emissions from NH(c’II) and NH(A311)were monitored at 326.0 f 0.2 nm and at 335.8 f 0.2 nm, respectively. were monitored a t 326.0 nm with a spectral bandwidth of AA = 0.2 nm. This emission originates from the Q(6) line and partly from the P(2), P(3), Q(S), and Q(7) lines. Since exponential decays were observed, the decay rate, T - ~was , determined from 7-l
=
7o-l
+ k~[”,]
(11)
where T~ is the zero-pressure fluorescence lifetime and kQ represents the effective quenching rate constant by the parent molecules including the photolysis p r o d ~ c t s .For ~ the emission from NH(c’II), Figure 4 displays a plot of 7-l vs. [HN,] from which we obtain T~ = 480 f 80 ns and kQ = (7.4 f 0.4) X cm3 s-I; all error limits, quoted in this paper, correspond to three times the standard deviation. On the other hand, single exponential decays were not observed for the triplet emission in the photolysis of HN,. A slowly decaying fluorescence component was found to be present 50 p s after the
3630 The Journal of Physical Chemistry, Vol. 88, No. 16, 1984
n = 1 .O for the singlet emission. For the triplet emission, n = 1.4 for the first 4 ps after the laser pulse (curve b) and n = 1.9 for the delayed time interval 17 to 50 ps (curve c). These and the following values of n were determined to be precise within 10%. CH3NHz. The emissions we observe in the ArF laser photolysis of CH3NH2are summarized in Table I. Only triplet emission was observed for NH in this system. The population ratio for the lowest vibrational levels of the (A311) state is N(u’=O)/N(u’=l) = 7. In addition, excited CH(C2Z+, B2Z-, A2A) and excited CN(B2Z+)radicals are formed. Under the present experimental conditions, the strongest emission originates from the CH(A2A) state, which was found to be two times more intense than that from NH(A311). At comparable pressures, the intensity of the triplet N H emission was observed to be about ten times weaker than in the case of NH3.s The emission from CN(B2Z+)was about ten times weaker than that from CH(AZA). The spectrum of the NH(A311,u’=O+X3Z-,~”=O) emission excited in the photolysis of methylamine is similar to that obtained in the photolysis of HN, except that high rotational levels (N’ I23) are less efficiently populated. An analysis of the relative rotational populations yields a non-Boltzmann distribution in this case. For large values of N’, Trotis about 2000 K. For 6 < N’ < 1 1, a leveling off for I/sNlv4is observed, indicating much larger values of Trotin this range. Rotationally resolved spectra of the CH(A2A+X211) emission and of the CN(BZZ+-+X2Z+) and CH(B2Z-+XZII) emissions were registered and are shown in Figures 6 and 7, respectively. No further analysis of these spectra has been attempted in the present work. The fluorescence intensities of the CN(BZZ+),NH(A311), and CH(A2A) emissions were measured and found to depend on the nth power of the laser intensity with n = 2.4, 1.7, and 1.8, respectively. Hence, none of these emissions is generated by an absorption step involving only one photon. Since recent measurements of the lifetime of NH(A311,u’=O) do not agree5 we have repeated our previous measurement,5this time photolyzing CH3NH2. The data obtained in this experiment is displayed in Figure 4. The fluorescence lifetime at zero pressure was determined to be T~ = 470 f 30 ns, in very good agreement with our recent value r0 = 480 f 40 ns;5 the effective quenching constant was determined to be kQ = (3.4 f 0.2) X cm3 s-l. A preliminary measurement of the lifetime of CH(AZA,u’=O) resulted in a value of TO = 520 50 ns and kQ = (6 f 1) X cm3 s-l. This value of T~ is in good agreement with a previous determination (543 f 10 ns).l2
I F
200 100
5c
20 10
5
2
B
nz1.9
1
5
50
10
100
I , / rnJcm-2 Figure 5. Double logarithmic plot of the fluorescence intensity, IF,vs.
the laser intensity, IL. The slopes, n,of the straight lines were determined to be (a) n = 1.0 for the production of NH(c’II) from HN3at 0.13 mbar (aperture duration 0 to 5 ps); (b) n = 1.4 for the production of “(A”) from HN3 at 0.13 mbar (aperture duration 0-4 ps); (c) n = 1.9 for the production of NH(A311) from HN3 at 0.13 mbar (aperture duration 17-50
p~).
laser pulse. Moreover, the decay rate becomes slower with increasing H N 3 pressure. Lifetime measurements appear to be meaningless in this system. The results shown in Figure 4 for the triplet emission were obtained in the photolysis of CH3NH2. The number of photons absorbed to generate the fluorescence emission was determined by plotting the fluorescence intensity, IF,vs. the laser intensity, Z, in a double logarithmic plot. Such plots are shown in Figure 5 for the singlet (curve a) and the triplet emission (curves b and c). The slope of the straight line was found to be I
C H ( A ~ A- x
I
Haak and Stuhl
*
I
I
2nI
10 I
L20
L25
L30
I
I
1
I
I
I
15
20
1 1 1 1 1 1 1 1 1 1 1 1
L35
Wavelength / nm Figure 6. Spectrum of the CH(A2A,u’=O+X211,u”=O) transition in the ArF laser photolysis of CH3NH2at 0.13 mbar. The aperture duration was 1 ps and the spectral resolution 0.2 nm.
The Journal of Physical Chemistry, Vol. 88, No. 16, 1984 3631
Formation of Excited N H Radicals
CN l B 2 1 ' - X 2 E * l
CH (B21--X211
I
1O;Ol
(3.3)
11.11
P
2
0 . L
L
8
6
6
10
8
12
10
&
-380
385
390
395
LOO
Wavelength / nrn Figure 7. Spectra of the CH(B2Z-,~"+X2II,~"=0) and the CN(B22++X2Z+,A~=0)transitions in the ArF laser photolysis of CH3NH2at 0.13 mbar. The aperture duration was 2 ps and the spectral resolution 0.2 nm. TABLE 11: ProDerties of the NH ( N D ) ( A 3 ~ . " = ~ X 3 2 - , y " = O ) Emission Generated in the ArF Photolyses of Various Molecules parent molecule ( r 0f 3u)/ns n TrdK kq/cm3 s-l "3 nonexponential 1.4 (0-4 ps) 1.9 (1 7-50 p ~ ) 4300 (slower at greater pressure of HN,) 1.7 2000 (N' > 11) (3.4 f 0.2) X CH3NH2 470 f 30 -4500 (6 < N ' < 1 1 ) 545 f 65 1.5 4000 (N' > 11) (5.5 f 0.15) X (- 1800)' N2H4 1.8 3000 ( N ' > 15) 4800 (7 IN' I14) 490 f 50 ( u = 0) -2 4500 f 500 390 40 ( u = 1) 1.9 740 ( N ' L 14) 480 f 40 "3 4400 (N' < 14) (4.4 f 0.4) X 435 f 40 1.9 550 ("2 16) (3.3 f 0.3) X ND3 3500 ( N ' < 16)
+
remarks this work this work ref 6, 13 this work ref 7 ref 5 ref 5
'Calculated from the data displayed in Figure 2 of ref 6. N2H4. The A r F excimer laser photolysis of N2H4 generates triplet emission of NH. The transitions observed are summarized in Table I. Relatively strong emission is observed from the first vibrational level of NH(A311) yielding the population ratio N(u'=O)/N(u'=l) = 5. In the rotationally resolved spectrum taken with an aperture duration of 7 ps, emissions from rotational levels up to N' = 28 are observed. Similar to the population obtained in the photolysis of CH3NH2, a non-Boltzmann distribution is observed for N,H,. For large rotational auantum numbers (N' 2 15), the distribution can be fitted to a temperature Trot= 3000 K. For lower quantum numbers (7 1 N ' 1 14) a leveling off for is observed, indicating much higher temperatures (4800 K). The fluorescence intensity of the NH(A311+X32-) emission was found to depend on the 13th power of the laser intensity, thus indicating that the absorption of at least two photons is necessary to generate an excited N H * molecule in the photolysis of hydrazine. No lifetime measurements were performed on this system.
Z/sNtv4
Discussion The A r F excimer laser photolysis of a number of molecules which yield emission from excited NH radicals has now been i n ~ e s t i g a t e d . ~ - ~ J ~The - ' ~ various emissions observed in these studies as well as those found in the current work are summarized in Table I. In addition to those molecules listed in Table I, the 193.3-nm photolysis of N H C O has been studied and yields NH(a1A).I6 Various experimental results for the excited triplet (12) J. Brzozowski, P. Bunker, N . Elander, and P. Erman, Astrophys. J., 207, 414 (1976). (13) N. Nishi, H. Shinohara, and I. Hanazaki, Chem. Phys. Lett., 73,473 (1980). (1 4) V. M. Donnelly, A. P. Baronavski, and J. R. McDonald, Chem. Phys., 43, 271 (1979). (15) J. B. Halpern, W. M. Jackson, and V. McCrary, Appl. Opt., 18, 590 (1979).
imidogen state observed in this study are collected in Table 11. Examination of the data in Tables I and I1 leads to the observation of several trends concerning the N H * formation in these systems. These observations will be considered further after a discussion of the results for the individual parent molecules. HN3. Fluorescence from excited N H radicals has been observed in the vacuum-UV photolysis of HN, by WelgeI7 and by Okabe.I8 Although the energy of one laser photon is sufficient for the reaction HN3 + hvL NH(c'II) + N2(X'Z,+) (1) to occur at wavelengths below 213.0 nm, the photolysis by UV light around 200 nm decomposes HN, mainly into NH(a'A) and N2(X1Zg+).19 (A similar result is observed for the 193.3-nm photolysis of the isoelectronic species HNC0.I6) The quantum yield for the formation of NH(c'II) has been determined by Okabe'* to be 10.02 in the vacuum-UV region. The yield decreases at longer wavelengths and is less than 7 X 10-4-at 184.9 nm.I8 The absorption coefficient of H N 3 has been measured to be uIg3= 2.4 X cm2 at 193 nm.I8 On this basis one calculates that about 11% of the H N 3 molecules are destroyed in the beam during the laser pulse. With the upper limit given for the fluorescence yield at 184.9 nm,18 a production per cm3 of less than 2 X 10" NH(c'rI) is calculated for the laser photolysis. This estimated value appears to be reasonable since a relatively intense fluorescence signal is observed. For the present experimental conditions, the yield for this spin-allowed process appears to be (16) W. S. Drozdoski, A. P. Baronavski, and J. R. McDonald, Chem. Phys. Lett., 64, 421 (1979). (17) K. H. Welge, J . Chem. Phys., 45, 4373 (1966). (18) H. Okabe, J . Chem. Phys., 49, 2726 (1968). (19) H. Okabe, "Photochemistry of Small Molecules", Wiley, New York,
1978.
3632 The Journal of Physical Chemistry, Vol. 88, No. 16, 1984 of the same order of magnitude as that for the formation of NH(A311) in the two-photon absorption by a m m ~ n i a .In ~ the spin-forbidden photodissociation process forming NH(A311) HN3
+ hvL
-
NH(A311)
+ N2(X’Zg+)
Saturation might also be the reason for a generally weaker dependence of the fluorescence intensity, IF,on the laser intensity, ZL, observed by Nishi et al., than observed in the present work. For example, these authors13 find a square dependence of the C N fluorescence intensity on the laser power while we observe n = 2.4 for the relationship IF ILn. Clearly, the absorption of at least three photons is necessary to yield the observed C N emission while, for the formation of the excited N H and C H radicals, at least two photons are required. The generation of these excited radicals (and of NH2*) has been previously attributed to the existence of a common superexcited p r e c ~ r s o r ’which ~ is difficult to reconcile with the different values for n measured in the present study. If spin conservation is assumed, the following processes are consistent with the energy requirements and with the observed laser intensity dependence:
-
(2)
an excess energy of about 18 000 cm-’ is available for distribution among the products. Although this energy is sufficient to excite the rotational levels of NH(A311) to more than N’ = 27 (1 1 300 cm-’), the data of Figure 5 indicate that the triplet emission is generated by a process consuming more than one photon. That is, at long delay times, the fluorescence intensity is found to depend almost on the square (n = 1.9) of the intensity of the laser. At shorter delays, a markedly weaker dependence (n = 1.4) was measured which might be due to the occurrence of both spinforbidden process 2 and a two-photon process generating N H (A311). Process 2 has been found by OkabeIs to be a minor channel. In the vacuum-UV region, reaction 2 occurs to an extent of less than 5% of reaction 1.18 Whether its efficiency increases at longer wavelengths is not known. Alternatively, an additional linear formation process of NH(A311) might be explained by the quenching of NH(c’II) leading partly to “(A%). Most likely, a process originating in the absorption of two photons dominates the generation of the NH(A311-+X32-) emission at long delay times. Since this process lasts for about 50 ps, it must be a reaction of either two photodissociation products or a product from a two-photon dissociation. Previously, Okabel* has also observed emission from NH(A311) in the H N 3 photolysis which he attributed to the reaction of electronically excited N2*, most probably N2(B311g),with HN3. Excited N2* might be formed in the spin-allowed two-photon photodissociation process H N 3 + 2hvL
-+
CH3NH2
-
-+
-+
CH3NH2
NH(X32-,A311) + Nz(A32,,,B311g) (3)
This process requires the energy of more than one laser photon. Excited triplet N2* molecules might then react with the parent molecule according to
+
-
+
N2*(A,B) HN, NH(A311) 2N2 (4) Okabet8has estimated the rate constant of this reaction to be on the order of cm3 s-’. The exothermicity of reaction 4 provides sufficient energy to explain the observed excitation of the rotational levels of “(A%). Literature values of the zero pressure lifetime of NH(clII), T ~ range from 400 to 500 ns. The present value ( T = ~ 480 f 80 ns) lies at the upper limit of this range. A very detailed previous determination20 has resulted in values of T~ for different rotational levels ranging from 41 1 f 4 ns for N’ = 2 to 226 f 5 ns for N’ = 17. Further experiments are necessary to decrease the error of the present study and to clarify whether systematic errors such as those caused by radiation trapping can increase the fluorescence lifetime in this system. The rate constant (kQ = (7.4 h 0.4) X cm3 s-l) measured for the quenching of NH(c’II) by H N 3 is very large. In this laser photolysis system, the products are formed in an appreciable concentration and can hence participate in the quenching process. It seems, however, to be unlikely that these products can have influenced the present very large value of k , by more than f15%. In summary, the processes leading to excited N H in the ArF laser photolysis of H N 3 were found to be very different from those observed in the photolysis of ammonia. CH3NH,. In addition to the emissions observed by Nishi et al.,6913we observed fluorescence from the transitions N H (A3n,u’=l-X3E-,u’’=O) and CH(C2E+,u’=O-+X2II,u‘’=O). While Nishi et a1.l3 have reported the N H triplet emission to be dominant, we observe the CH(A2A) fluorescence to be the most intense emission. This difference in the rclative intensities is most likely due to saturation occurring in the experiment of Nishi et al.,13 since they have irradiated CH,NH, with a softly focused laser beam delivering a power of 50 MW/cm2. This irradiance is about ten times larger than that in the present experiment. (20) W. H. Smith, J. Brzozowski, and P. Erman, J . Chem. Phys., 64,4628 (1976).
Haak and Stuhl
,
+ 2hvL N H * + CH3 + H CH*
+ N H 2 + H,
CH*
+ NH, + H
-
+ 3hvL
CN*
A,H = 11.32 eV
A,H = 10.97-12.14 eV A,H = 11.04-12.21 eV
+ H2 + 3H
(5)
(6) (7)
A,H = 14.57 eV (8)
For reactions 6 and 7 the given ranges of values for ArH takes into account the different electronic states observed for CH. The generation of superexcited states13 demands sequential absorption of two or more photons by the parent molecule. Unfortunately, the present experiments do not allow us to distinguish this mechanism from a two-step process in which the primary products such as CH3NH, CH2NH2,and CH,=NH are consecutively excited during the laser pulse. Conservation of energy requires that the primary products in the two-step process possess a large fraction of the excess energy from the first photodissociation process. For example, to explain the formation of NH(A311,u’=0,N’=23), the primary products must provide almost 2 eV of their internal energy for the formation of the excited N H radicals. Although, for both methylamine and ammonia, the excited NH* is formed in a two-photon process, the rotational populations in the excited state are observed to be different. Particularly, the truncation of the rotational population found for ammonia is not seen for CH3NH2. The reason for this difference might be the larger energy excess for rotational excitation available in the photolysis of CH3NHz. An apparent discrepancy is noted between the present rotational distribution (T,,, = 2000 K for N’ I1 1) and that reported recently (T,,, = 4000 K for N ’ I 11).6 From the data displayed in Figure 2 of ref 6, however, we calculate T,,, = 1800 K in good agreement with the present value. The present value of the lifetime of NH(A311,u’=O) (ro= 470 f 30 ns) is somewhat smaller than that reported by Nishi et aL6 for the same photolysis system (ro= 545 f 65 ns), but larger than the values reported by Smith et aLZ0 Also, the quenching rate constant determined in the present study for the addition of CH3NH2(kQ = (3.4 f 0.2) X cm3 s-I) is smaller than that reported by Nishi et a1.6 (kQ = (5.5 f 0.15) X 1Olocm3 s-l). This difference is most likely caused by the higher laser intensity used in the experiment of Nishi et aL6 The absorption coefficient of C H 3 N H 2has been reported to be ~ 1 9 3= 1.8 X cm2 at 193 nrn.,l Hence, about 9% of the parent molecules are destroyed in the laser beam to yield primary products. The concentration of these photolysis products is too low to dominantly influence the very large value of kQ measured. However, at the large irradiance used in the focused beam of the previous experiments,6 about half of the parent molecules will be destroyed. The participation of these products in the quenching of N H * might have resulted in the larger value of kQ reported previously.6 Obviously great care has to be taken in studies of (21) E. Tannenbaum, E. M. Coffin, and A. J. Harrison, J . Chem. Phys., 21, 311 (1953).
Formation of Excited N H Radicals
The Journal of Physical Chemistry, Vol. 88, No. 16, I984 3633
quenching by (and of reactions with) species which absorb laser radiation. N,Hp. The present study confirms most of the results reported by Hawkins and Houston.7-However, we do not observe emissions at wavelengths above 405 nm, most likely because of the short aperture duration used in this work (1 p s ) . Furthermore, the non-Boltzmann distribution observed in the present work for NH(A311,v’=O) (Trot= 3000 K for N’I 15 and Trot= 4800 K for 7 IN’ I14) deviates somewhat from the reported single temperature Trot= 4500 f 500 K.7 This deviation can be partly explained by the additional observation of higher and lower rotational levels in the present study. Hawkins and Houston7 have determined a relatively large population ratio N(v’= l)/N(v’=O) = 0.67 by using the intensities of the (1,O) and (0,l) bands. Using the intensities of the unresolved Q branches of the (1,l) and (0,O) bands we calculate a ratio of 0.2. This is the largest vibrational population we have so far observed for v’ = 1 in the A r F laser photolyses of NH3, ND3, N2H4, CH3NHz, and HN,. The difference in the population ratio between the present and the literature value7 is probably due to uncertainties in the FranckCondon factors used which are probably less accurately known for bands with Av # 0. Using laser-induced fluorescence to monitor NHz(X), Hawkins and Houston have shown that N H z is not a major primary product of the laser photolysis of hydtazine and suggest that hydrazine absorbs ArF laser light to form hydrazyl radicals and H atoms7 N2H4 hVL NzH3 H (9)
-
+
+
These authors further suggest that excited triplet N H is subsequently generated in the photolysis of NzH3 NzH3
+ hVL
-
NH2
+ NH(A311)
(10) Energy conservation necessitates for the generation of highly excited NH(A311) radicals with N ’ I 28 that the hydrazyl radicals must be formed in reaction 9 with internal energies of up to 12 100 cm-’. Hawkins and Houston’ proposed that these hot radicals have been observed in previous flash photolysis experiment^^^^^^ by their continuous UV absorption at 290 nm. In previous vacuum-UV flash photolysis experiments, one of us has also observed continuous absorption in the 250-340-nm region when photolyzing N2H4 at pressures above 0.13 mbar in the absence of inert gas.24 This absorption was observed to decay very slowly and was still present after 10 s when photolyzing N2H4 at a pressure of 2.7 mbar. It appears to be unlikely that the species causing such a long-lived absorption is hot hydrazyl. Excited triplet N H might be also generated by sequential absorption of two photons by the parent molecule (22) D. Husain and R. G . W. Norrish, Proc. R. SOC.London, Ser. A , 273, 145 (1963). (23) M. Arvis, C. Devillers, M. Gillois, and M. Curtat, J . Phys. Chem., 78. 1356 f1974). (24) F,‘ Stuhl, Dissertation, Universitaet Bonn, 1966.
NzH4
-
+ 2h~L
+ H (1 1) NH(X3Z-) + NH(A311) + Hz (12)
NH2(XZB1) + NH(A311)
The excess energy available for both reactions (21 700 cm-’ for reaction 11 and 25 100 cm-’ for reaction 12) is more than sufficient to excite N’ = 28 (12 100 cm-l). As for the other compounds investigated, we can thus not decide which process forms the observed N H fluorescence on the basis of current data.
Comparison and Conclusions The formation of excited imidogen was observed in the ArF laser photolysis of HN3, CH3NHz,and NzH4. A comparison of the present results and those obtained recently for NH3 and ND35 (see Tables I and 11) reveals several interesting features. All these molecules, with the exception of HN3, produce strong NH(A311) fluorescence in a two-photon photodissociation process. Hydrazoic acid is the only parent molecule observed to yield excited NH in a one-photon process. Moreover, excited NH(A311) is formed mainly by reaction in this system. (In addition, the spin-forbidden process 2 and/or the two-photon process 3 might weakly contribute to the triplet emission.) The N H (ND)(A311,v’=O) generated in two-photon processes from NH3, ND3, NzH4, and CH3NH2exhibit characteristic rotational distributions. A common feature of all these distributions is a relatively high rotational temperature of roughly 4500 K at low rotational quantum numbers, N’. For high values of N’, the rotational population is found to depend on the excess energy available from the two-photon process. Being exceptional cases of two-photon resonance processes, the ArF photolyses of N H 3 and ND3 result in marked truncations of the rotational populations because of energy defi~iency.~The largest excess energy is available for NzH4 (2.7 eV) and the highest rotational temperature of -3000 K is observed for large N’. Moreover, the largest relative population of the first vibrational state NH(A311,v’= 1) was measured for NzH4 It is noteworthy that all these four parent molecules contain N H z which might break off in the first photolysis step such that the second photolysis step represents the photolysis of N H z fragments. However, neither for NzH, nor for CH3NH2has the primary formation of N H z been proven. At any rate, if a minor photolysis channel produces this radical it has to be formed with large internal energy to explain the rotational excitation in the two-step photolysis. Finally, it should be mentioned that these photolysis systems represent a convenient tool for the study of properties of excited imino radicals. Work in this area is in progress in our laboratory. Acknowledgment. Financial support by the Deutsche Forschungsgemeinschaft (SFB 42) and by the Fonds der Chemischen Industrie is gratefully acknowledged. We thank Dr. R. D. Kenner for helpful suggestions during the preparation of the manuscript. Registry No. HN?, 7782-79-8: CH,NH,. 74-89-5: N,H,. 302-01-2: NH, i3774-92-0; cH, 331 5-37-5; CN; 2074-87-5. -
1
