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Time resolved observations of NH2 and benzyl radicals produced in the infrared multiple photon dissociation of benzylamine
Voolume99, number 5,6
CHEMICAL PHYSICS LETIERS
TIME RESOLVED PRODUCED H. REISLER.
OBSERVATIONS
OF NH, AND BENZYL RADICALS
Ih’ THE INFRARED MULTIPLE PHOTON DISSOCIATION OF BENZYLAMINE * F.B_T_ PESSINE ** and C. WITTIG
Chernisrr_r Deparfmmr.
lhirersir~
of SourArm
Lbiifornia
Los Angeies.
cirlifomia
90089-0484.
US-4
Received 13 Mag 1983
T11caverage internal energy of benzylamine molecules dissociated via infrared multiple photon excitation is derived from laser induced fluorcsccnce (LIF] measurements of [Nf-12] versus time, and is SO-70 kcal mol-’ above threshold at fluences of IO-30 J cm-Z_ Ihc benzyl radical is monitored by LIF and is produced vibrationally “hot*‘_ f;luorescence lifet-mzesaf bands in its 1 2.\2 - 1 zB2 system are reponcd.
I _ Introduction Infrared
multiple
photon
excitation
(IR MPE) is available for infrom the ground elec-
currently the most generai technique
ducing unjmolecu~ar
reactions
tronic srstc under collision-free conditions. It offers disrinzt advantages over themlal activation, in that the reaction can be followed in real time, and nascent
energy dlstrlbutions in the fragments can be obtained, In contrast to the growing number of studies concerned with energy disposal among the fragments reports of time resolved mea[I--%] * , esprrimental suremems of unimolecular reaction rates in the absence of collisions ale scarce [2,4,5] _Such studies are important if we are to develop an tlnderstanding of energy disposal in the fragments. since they yield directly the average energy from which dissociation occurs. It is not straightforward to measure unimolecular reaction ram using infrared multiple photon dissociation (IK MPD), since product formation often occurs during the CO? laser pulse, and thus the reaction rate is convolurcd \%tli the lsser pulse shape. This is par* Kesc~rch supporred by the US Air Torte Office of Scientific Research. ** Prmlanenr address: lnstituto de Quimica, Universidade Estadual de Campinas, Campinas. S9_. Bra&_ * FGxrecent rewews see refs. f X.2]_
3SS
ricularly true for small molecules, since reactions are fast even at modest “excess” energies above reaction threshold. Dissociation of large molecules using low laser fluences, however, should be suffcienrly slow to enable the separation of dissociation from the laser pulse shape. RRKM calculations suggest that for medium-size molecules with high acrivatlon energies, reaction rates
here was to determine directly the average excess energy achieved in IR MPE as a function of laser ffuence. Benzylamine (BA) was chosen, since it undergoes a simple bond fission reaction terminating in two ground-state radicals: r1ku
C,H,CH2NH2
-
CgH5CH2 + NH,,
,&I@ = 712 lical mol-l_
(1)
Moreover, both fragments can be monitored via laser induced fluorescence (LIF). The NH7 radical is particularly well suited to serve as a probe for BA dissociation, since it is expected to carry only-a small fraction of the available energy, and its detection is thus simplified. Also, its open rotational structure is amenable to determining product energy distributions. The thermal activation of benzylamlne has been studied previously [a], and the high-pressure 0 009~2614/83/0000-0000/S
03.00 Q 1983 North-Holland
Volume 99. number 5,6
CHEMICAL PHYSICS LETTERS
Arrhenius parameters are available. Thus, a correlation between the unimolecular reaction rate and the excess energy is straightforward.
2.
Experimental
19 Aumt
1983
rescence at =500 nm with a Kodak no. 40 filter. Fluorescence was detected along an axis perpendicular to both laser beams with a photomultiplier tube @‘MT). Signals from the PMT were amplified, digitized, and stored and averaged yith a computer which also controlled the timing of the lasers and the scanning of the dye laser.
The experimental arrangement has been described in detail previously [3,4,7], and therefore only a brief description will be given here. BA vapor (1-S mTorr)l was passed slowly through an Al chamber, where unimolecular reaction was effected via IR MPD using the focused output from a CO, TEA Laser (Lumonics 103, 150 ns fwhm). The laser was operated on the 1033 cm-l line [(OOl)-(02O)P34], which coincides with an absorption feature in BA [8] _The laser was operated with almost no N2 in order to eliminate the “tail” of the pulse, and we estimate that less than 5% of the energy was in the tail. The beam was brought to a focus at the center of the chamber with a 25 cm focal length lens, and the beam size at the focus was estimated using a series of apertures_ 80% of the laser energy was concentrated uniformly in an area of 05 + 0.1 mm2, and this energy was varied using CaF2 attenuators. The fluence was estimated from the measured pulse energies and beam size, and although the actual value of the fluence may differ by as much as 30% from our estimate, due to difficulties in measuring accurately the beam size, the relative changes are accurate to within 5%. Great care was taken to ensure that fluctuations and long-term variations in the CO, output energy were very small, since
3.
at low fluences a small variation in laser fluence results in a large change in fragment yield. To achieve
tional line. In addition, appearance times obtained using LIF of the (0,15,0) + (0, 0,O) band were identical_ The average dissociation rate of BA was derived by fitting the time resolved data to a two-exponential rise and decay function. For each laser fhrence, the decay portion, which reflects mainly the movement of NH2 radicals, was measured at long delay times and fitted to a single exponential_ The rise portion was then fitted using the measured decay rate_ The fit was rather good at low fluences, where most of the fragments appear after the termination of the CO, laser pulse (see fig. l), and the dissociation rates are summarized in table 1_ Although we did not try to derive rotational temperatures for NH2, comparisons between the present
such stability, the laser was operated for several hours prior to the measurements. LIF spectra were obtained using a Nd:YAG laser pumped dye laser. The dye and CO, laser beams intersected at 90” in the center of the chamber, and the diameter of the dye laser beam was ==3 mm_ The delay between the lasers was varied with a digital delay generator (10 11sresolution, 525 ns jitter)_ NH, was detected by exciting the (0,10,O) -+ (O,O, 0) or the (0,15,O) -+ (O,O, 0) bands of the 2A1 +- *B1 system [9-l 11, and observing the fluorescence with Coming 2-62 and 3-7 1 cut-off filters respectively. The benzyl radical was monitored by exciting in the 1 2A2 f 1 2B2 system [12-171 near 447 nm, and observing the fluo-
Results
Appearance times of NH, were obtained by monitoring the intensity of the LIF signal as a function of the delay time between the two lasers. Several rotational lines, mainly in the strong RQo v branch of the (0,10,O)+ (0, 0,O) band of the 1 A, + ‘B, systern were used [lo] _Measurements were done at low pressures (1-2 mTorr) in order to minimize relaxation, and at these pressures the hard sphere collision rate is ~104 s-l molecule-l: In addition, we found that the appearance times were independent of the rotational level probed (2,4, or 6), even at the lowest fluences, and that the NH, signal was linear with the probe laser intensity_ Typical results, obtained at different CO2 laser fluences, are shown in fig_ 1_ The appearance times vary markedly witil fhrence, and at the low iluences are much longer than the laser pulse duration_ At high fluences, the rise time is convohrted with the temporal profile of the CO2 laser, as expected_ Although the fluctuations in the signals obtained at low fluence are rather high, the results were reproducible from day to day, and did not vary with the NH, rota-
389
CHEMICAL PHYSICS LETTERS
Volume 99. number 5.6
Co,
LASER
19 August 1983
PULSE
150 ns-
10 J
m-n-*
Ii
I:ig. 1. NH, appearance times following the IR MPD of BA at different Iaser fluences. The CO1 laser pulse is dispiayed in the up Per left. and each point is the average of 8 laser fiigs. The parent pressure is l-2 mTorr. The circles represent data and the solid lines are obtained using a singe dissociation rate (see text).
nascent NH2 LIF spectra, spectra obtained using high buffer gas pressures in order to thermalize rotations, and spectra of nascent NH, produced in the IR MPD of methylamine (for which a rotational temperature of -100 + 20 K was derived [ 1S]) indicate that the NH2 spectra from BA are somewhat “hotter.’ than room temperature. but not by much. We made no attempt to detect escited vibrational levels in the present work.
We also tried to obtain LIF spectra of the benzyl radical, whose spectrum was previously observed in the 433-455 nm region, and assigned to the 1 ?A2 + 1 “B2 transition [12-171. Nothing which could be identified as a benzyl spectrum was obtained at pressures of l-10 mTorr_ Only very weak features, probably rotational lines of the (0, 15,O) ~(0, 0,O) band in NH,, were observed_ However, upon addition
Table 1 Unimolecular reaction rates of benzyiamine Laser fluencc
(J cm”) 10
13 15 20 27 48
Rate (105 s-1)
Lit-&me (Ils)
“Excess” energy a) (IrcaImol-*)
“Exess” CO Ia& photons2a)
3.4 5.0 7.7 14.0 50 ,100
2.9 2.0 1.3 0.7 0.2 co.1
50 55 58 62 72 >78
17 19 20 22 25 >27
2 0.5 z 1.0 + 1.2 -z 2.0 * 10
+ o-5 2 0.4 %0.2 + 0.1 + 0.05
a’ “Excess” refers to the eneqg in escess of reaction threshold.
390
CHEMICAL
Volume 99, number 5,6
SmTorr 1.5 Torr
WAVELENGTH
19 August-1983
PHYSICS LETTERS
benzylomine Ar
1 run)
Fig. 2. LIF spectrum of the benzyl radical produced by IR MPD of benzylamine; 15 Torr of Ar is used to relax the nascent excitation. The benzylamine pressure is 5 mTorr, and the delay between the CO2 and dye lasers is 30 JIS.
of 15 Torr of Ar, a typical spectrum of the benzyl radical, whose intensity peaked at 30 ps delay, was readily observed and is displayed in fig. 2. The same spectrum was also obtained following the IR MPD of toluene and benzylchloride in the presence of Ar_ The fluorescence lifetimes (extrapolated to zero pressure) of the A1 level (447.7 nm) and A2 level (446.5 nm) were 1 S i 0.2 and 1.6 + 0.3 p respectively, in good agreement with lifetimes obtained in a matrix [I9-211, and in the gas phase [22] _Our results differ from those obtained in the gas phase by Okamura et al. [ 151, and the reason for this discrepancy is not clear.
CO, laser beam that we probe is not uniform. This should not be important at low fluences, where the fragment yields increase very rapidly with laser energy, and therefore the signals arise mainly from the high intensity part of the CO, laser pulse. The changes in the average dissociation rate with fluence (fig_ 1) are too large to be caused by errors due to the inherent averaging of the experiment_ An important feature of the experiment is the short duration of the CO, laser pulse compared with the appearance times of the fragments produced at low laser fluences. Thus, the appearance times that we measure reflect the microscopic unimolecular reaction rates, rather than steady state rates (as derived by Quack [23], for example) which result from a convolution of the optical pumping rates and the reaction rates during the CO, laser pulse_ The latter are phenomenological values, specific to IR MPE, whereas the former are the true (albeit average) unimolecular reaction rates. It is therefore possible to relate our measured reaction rates to the average energy in the
4
BENZYLAMINE lo’-
: ”
4_ Discussion It is important to emphasize that the dissociation rates reported here are average values which derive from an unknown distribution of excess energies achieved via IR MPE. The width and shape of this distribution are unknown, but model calculations indicate that it is narrower than a thermal distribution [2] _The accuracy of our low-fluence measurements is insufficient to justify fitting the appearance times to a distribution of reaction rates, and thus only average values are derived. In addition, with our experimental geometry, the spatial intensity profile of the
IO5 40
50 EXCESS
60 ENERGY
70
80
! 0
(kc01 mol-‘)
Fig_ 3. Unimolecular reaction rate of BA versus energy in excess of the threshold energy, calculated according to ref. 1241 using Am, Earn. and vibrational frequencies from ref. [6]_ The brackets indicate the range that was observed in the IR hlPD of BA.
391
Volume 99, number 5.6
CHEMICAL PHYSICS LETTERS
dissociating BA molecules using thermal activation data [6]. The unimolecular reaction rate at energy E can be extracted from the thermal average as per Forst [24, 251, and the results of such a deconvolution are shown in fig. 3. We use the frequency factor A,, activation energy Ea_ and vibrational frequencies of BA from ref. [6], and we calculate densities of states according fo ref. [26]_ A similar curve is obtained using RRKM theory [25,26] with vibrational frequencies for the molecule and the activated comples from ref. (61 * _From this curve, we can estimate the average energy in the dissociating BA molecule [6], and hence the number of “excess” CO, laser photons absorbed. These estimates are summarized in table 1. and show that when the fluence changed from 10 to 30 J cm-2, the average number of excess photons absorbed increased from 17 lo 75. At low fluences, the excess energy is apparently fluence dependent. since most of the dissociation occurs after the termination of the CO, laser pulse. At higher fluences, however, it was shown that the energy above reaction threshold is intensity dependent. and is controlled by the competition between the net optical pumping rate and the reaction rdte [I ,2,28-301. From table 1 it is evident that the energy available for fragment excitation is rather high. Most of this energy will reside in the benzyl radical, since it has nwny more vibrational degrees of freedom than NH,. and thus, rile benzyl radical is produced vibrationally “hot”. This esplains why a proper LIF spectrum of the brnzyl radical is observed only at high buffer gas pressures and long delays_ when relaxation of the ndsccnf vibrarional excitation insures that a structured benzyl spscrrum is observed_
5. Conclusion The average energy above dissociation threshold, following the IR hlPE of benzylamine was derived from measurements of the appearance times of the Nl IL fragment at low CO, laser fluences. This energy depends on the CO, laser fluence, and at IO-30 J cm-‘, It varied from 50 to 70 kcal mol-l above disso* l-or 3 discussion of the convolution of thermal activation dsta see &o ref. 1371. 392
19 Au,mt 1983
ciation threshold. As expected, most of this ener7 resides in the benzyl radical, which is produced vibrationally “hot”. Measurements of “excess” energies via IR MPD are important in studies concerned with enerM disposal in fragments and their comparisons with statistical theories. Such studies are now possible for NH, from BA, since its spectrum, although complex, has been rather well analyzed_
Acknowledgement
We wish to thank G. Hancock for unpublished results on the LIF of NH,, and G. Smith for unpublished results on the lifetimes of the benzyl radical.
References [l] hi .N.R. Ashfold and G. Hancock, Chem. Sot. Spec. Res. Rep. 4 (1981) 73; P-A. Schulz, AaS. Sudbo, 1-J. Krajnovich, H.S. Kwok, Y-R. Shen and Y-T. Lee, Ann. Rev. Phys. Chem. 30 (1979) 379. [ 21 D.S. King, in: Dynamics of the excited state, ed. KP. Lawley (WiIey, New York. 1982) p_ 105 and references therein. [3 ] IL Reisler, F. Kong, A_hl. Renlund and C. Wit@, J. Chem. Phys. 76 (1982) 997. [4] H. ReisIer, F. Kens, C. Wittig, J. Stone, E. Thiele and hl.Iz. Goodman, 1. Chem. Phys. 77 (1982) 328. [5] S. Ruhman, 0. Anner and Y. Haas, J. Chem. Sot. Faraday Disc. 75 (1983) to be published. f6] D.M. Golden, R3i. SoBy. N_A_ Gac and S-W_ Benson, J. Am. Chem. Sot. 94 (1972) 363. [7] H. Reisler, M. hlangir and C. Wittig, J. Chem. Phys. 47 (1980) 49. [8] I. Welti, Infrared vapor spectra (Heiden, London, 1970). [9] I;. Dressier and D.A. Ramsay, Phil. Trans. Roy. Sot. A251 (1959) 553. [lo] 11. KroIl, J. Chem. Phys. 63 (1975) 319. [ 111 A-J. Roberts and G. Hancock, unpublished results. [12] G_ Porter and B. Ward, J. Chim. Phys. 61 (1964) 102. [13] C. Cossart-hlajos and S. Leach, J. Chem. Phys. 56 (1972) 1534;64 (1976) 4006. [141 D-hi. Brenner, G.P. Smith and R.N. Zare, J. Am. Chem. Sot. 98 (1976) 6706. 1151 T. Okamura, T.R. Charlton and B.A. Thrush, Chem. Phys. Letters 88 (1982) 369. 1161 H-H. Nelson and J-R. McDonald, J. Phys. Chem. 86 (1982) 1242. 1171 hl. Heaven, L. Dimauro and TA. hliller, Chem. Phys.
Letters 95 (1983) 347.
Volume 99, number 5,6
CHEMICAL PHYSICS LEtiERS
1181 M-N-R. A&fold, G. Hancock and A-J. Roberts, Faraday Disc. C&em. Sot. 62 (1979) 204. [19] T. Okamnra and T-Tanaka, J_ Phys. Chem. 79 (1975) 2728. 1201 T. Okarnura, Ii. Obi and I. Tanaka, Chem. Phys. Letters 26 (1974) 218. [21] J-D. Laposa and V, Morrison, Chem. Phys. Letters 28 (1974) 270. 1221 Dhl. Brenner, G-P. Smith and R.N. Zare, unpublished results. [23] hf. Quack, in: Dynamics of the excited states, ed. K.P. Iawley (Tsirey, New York, 1982) p. 39.5. 1241 W. Forst, J, Phys. Chem. 76 (1972) 342.
19 Au__rmst 1983
f25] W. Forst, Theory of urdmoleeular react (Academic Press, New York, 1973)_ 1261 PJ. Robinson and K.A. Holbrook, UnimolecuJarreactions Wiley, New York, 1972). [27] H. Reisler, F&T. Pessine, Y. Haas and C. Wittig, J. Chem Phys. 78 (1983) 378. [58] CM. Miller and R.N. Zare, Chem. Phys. Letters 71 (1980) 376. [29] A.M. Renlund, H. Reisler and C. Wttig, Chem. Phys. Letters 78 (1981) 40. 1301 X1.N.R. AsMold, G. Hancock and hi_ Hardrake, 3. Photothem. 14 (1980) 85,
393
CHEMICAL PHYSICS LETIERS
TIME RESOLVED PRODUCED H. REISLER.
OBSERVATIONS
OF NH, AND BENZYL RADICALS
Ih’ THE INFRARED MULTIPLE PHOTON DISSOCIATION OF BENZYLAMINE * F.B_T_ PESSINE ** and C. WITTIG
Chernisrr_r Deparfmmr.
lhirersir~
of SourArm
Lbiifornia
Los Angeies.
cirlifomia
90089-0484.
US-4
Received 13 Mag 1983
T11caverage internal energy of benzylamine molecules dissociated via infrared multiple photon excitation is derived from laser induced fluorcsccnce (LIF] measurements of [Nf-12] versus time, and is SO-70 kcal mol-’ above threshold at fluences of IO-30 J cm-Z_ Ihc benzyl radical is monitored by LIF and is produced vibrationally “hot*‘_ f;luorescence lifet-mzesaf bands in its 1 2.\2 - 1 zB2 system are reponcd.
I _ Introduction Infrared
multiple
photon
excitation
(IR MPE) is available for infrom the ground elec-
currently the most generai technique
ducing unjmolecu~ar
reactions
tronic srstc under collision-free conditions. It offers disrinzt advantages over themlal activation, in that the reaction can be followed in real time, and nascent
energy dlstrlbutions in the fragments can be obtained, In contrast to the growing number of studies concerned with energy disposal among the fragments reports of time resolved mea[I--%] * , esprrimental suremems of unimolecular reaction rates in the absence of collisions ale scarce [2,4,5] _Such studies are important if we are to develop an tlnderstanding of energy disposal in the fragments. since they yield directly the average energy from which dissociation occurs. It is not straightforward to measure unimolecular reaction ram using infrared multiple photon dissociation (IK MPD), since product formation often occurs during the CO? laser pulse, and thus the reaction rate is convolurcd \%tli the lsser pulse shape. This is par* Kesc~rch supporred by the US Air Torte Office of Scientific Research. ** Prmlanenr address: lnstituto de Quimica, Universidade Estadual de Campinas, Campinas. S9_. Bra&_ * FGxrecent rewews see refs. f X.2]_
3SS
ricularly true for small molecules, since reactions are fast even at modest “excess” energies above reaction threshold. Dissociation of large molecules using low laser fluences, however, should be suffcienrly slow to enable the separation of dissociation from the laser pulse shape. RRKM calculations suggest that for medium-size molecules with high acrivatlon energies, reaction rates
here was to determine directly the average excess energy achieved in IR MPE as a function of laser ffuence. Benzylamine (BA) was chosen, since it undergoes a simple bond fission reaction terminating in two ground-state radicals: r1ku
C,H,CH2NH2
-
CgH5CH2 + NH,,
,&I@ = 712 lical mol-l_
(1)
Moreover, both fragments can be monitored via laser induced fluorescence (LIF). The NH7 radical is particularly well suited to serve as a probe for BA dissociation, since it is expected to carry only-a small fraction of the available energy, and its detection is thus simplified. Also, its open rotational structure is amenable to determining product energy distributions. The thermal activation of benzylamlne has been studied previously [a], and the high-pressure 0 009~2614/83/0000-0000/S
03.00 Q 1983 North-Holland
Volume 99. number 5,6
CHEMICAL PHYSICS LETTERS
Arrhenius parameters are available. Thus, a correlation between the unimolecular reaction rate and the excess energy is straightforward.
2.
Experimental
19 Aumt
1983
rescence at =500 nm with a Kodak no. 40 filter. Fluorescence was detected along an axis perpendicular to both laser beams with a photomultiplier tube @‘MT). Signals from the PMT were amplified, digitized, and stored and averaged yith a computer which also controlled the timing of the lasers and the scanning of the dye laser.
The experimental arrangement has been described in detail previously [3,4,7], and therefore only a brief description will be given here. BA vapor (1-S mTorr)l was passed slowly through an Al chamber, where unimolecular reaction was effected via IR MPD using the focused output from a CO, TEA Laser (Lumonics 103, 150 ns fwhm). The laser was operated on the 1033 cm-l line [(OOl)-(02O)P34], which coincides with an absorption feature in BA [8] _The laser was operated with almost no N2 in order to eliminate the “tail” of the pulse, and we estimate that less than 5% of the energy was in the tail. The beam was brought to a focus at the center of the chamber with a 25 cm focal length lens, and the beam size at the focus was estimated using a series of apertures_ 80% of the laser energy was concentrated uniformly in an area of 05 + 0.1 mm2, and this energy was varied using CaF2 attenuators. The fluence was estimated from the measured pulse energies and beam size, and although the actual value of the fluence may differ by as much as 30% from our estimate, due to difficulties in measuring accurately the beam size, the relative changes are accurate to within 5%. Great care was taken to ensure that fluctuations and long-term variations in the CO, output energy were very small, since
3.
at low fluences a small variation in laser fluence results in a large change in fragment yield. To achieve
tional line. In addition, appearance times obtained using LIF of the (0,15,0) + (0, 0,O) band were identical_ The average dissociation rate of BA was derived by fitting the time resolved data to a two-exponential rise and decay function. For each laser fhrence, the decay portion, which reflects mainly the movement of NH2 radicals, was measured at long delay times and fitted to a single exponential_ The rise portion was then fitted using the measured decay rate_ The fit was rather good at low fluences, where most of the fragments appear after the termination of the CO, laser pulse (see fig. l), and the dissociation rates are summarized in table 1_ Although we did not try to derive rotational temperatures for NH2, comparisons between the present
such stability, the laser was operated for several hours prior to the measurements. LIF spectra were obtained using a Nd:YAG laser pumped dye laser. The dye and CO, laser beams intersected at 90” in the center of the chamber, and the diameter of the dye laser beam was ==3 mm_ The delay between the lasers was varied with a digital delay generator (10 11sresolution, 525 ns jitter)_ NH, was detected by exciting the (0,10,O) -+ (O,O, 0) or the (0,15,O) -+ (O,O, 0) bands of the 2A1 +- *B1 system [9-l 11, and observing the fluorescence with Coming 2-62 and 3-7 1 cut-off filters respectively. The benzyl radical was monitored by exciting in the 1 2A2 f 1 2B2 system [12-171 near 447 nm, and observing the fluo-
Results
Appearance times of NH, were obtained by monitoring the intensity of the LIF signal as a function of the delay time between the two lasers. Several rotational lines, mainly in the strong RQo v branch of the (0,10,O)+ (0, 0,O) band of the 1 A, + ‘B, systern were used [lo] _Measurements were done at low pressures (1-2 mTorr) in order to minimize relaxation, and at these pressures the hard sphere collision rate is ~104 s-l molecule-l: In addition, we found that the appearance times were independent of the rotational level probed (2,4, or 6), even at the lowest fluences, and that the NH, signal was linear with the probe laser intensity_ Typical results, obtained at different CO2 laser fluences, are shown in fig_ 1_ The appearance times vary markedly witil fhrence, and at the low iluences are much longer than the laser pulse duration_ At high fluences, the rise time is convohrted with the temporal profile of the CO2 laser, as expected_ Although the fluctuations in the signals obtained at low fluence are rather high, the results were reproducible from day to day, and did not vary with the NH, rota-
389
CHEMICAL PHYSICS LETTERS
Volume 99. number 5.6
Co,
LASER
19 August 1983
PULSE
150 ns-
10 J
m-n-*
Ii
I:ig. 1. NH, appearance times following the IR MPD of BA at different Iaser fluences. The CO1 laser pulse is dispiayed in the up Per left. and each point is the average of 8 laser fiigs. The parent pressure is l-2 mTorr. The circles represent data and the solid lines are obtained using a singe dissociation rate (see text).
nascent NH2 LIF spectra, spectra obtained using high buffer gas pressures in order to thermalize rotations, and spectra of nascent NH, produced in the IR MPD of methylamine (for which a rotational temperature of -100 + 20 K was derived [ 1S]) indicate that the NH2 spectra from BA are somewhat “hotter.’ than room temperature. but not by much. We made no attempt to detect escited vibrational levels in the present work.
We also tried to obtain LIF spectra of the benzyl radical, whose spectrum was previously observed in the 433-455 nm region, and assigned to the 1 ?A2 + 1 “B2 transition [12-171. Nothing which could be identified as a benzyl spectrum was obtained at pressures of l-10 mTorr_ Only very weak features, probably rotational lines of the (0, 15,O) ~(0, 0,O) band in NH,, were observed_ However, upon addition
Table 1 Unimolecular reaction rates of benzyiamine Laser fluencc
(J cm”) 10
13 15 20 27 48
Rate (105 s-1)
Lit-&me (Ils)
“Excess” energy a) (IrcaImol-*)
“Exess” CO Ia& photons2a)
3.4 5.0 7.7 14.0 50 ,100
2.9 2.0 1.3 0.7 0.2 co.1
50 55 58 62 72 >78
17 19 20 22 25 >27
2 0.5 z 1.0 + 1.2 -z 2.0 * 10
+ o-5 2 0.4 %0.2 + 0.1 + 0.05
a’ “Excess” refers to the eneqg in escess of reaction threshold.
390
CHEMICAL
Volume 99, number 5,6
SmTorr 1.5 Torr
WAVELENGTH
19 August-1983
PHYSICS LETTERS
benzylomine Ar
1 run)
Fig. 2. LIF spectrum of the benzyl radical produced by IR MPD of benzylamine; 15 Torr of Ar is used to relax the nascent excitation. The benzylamine pressure is 5 mTorr, and the delay between the CO2 and dye lasers is 30 JIS.
of 15 Torr of Ar, a typical spectrum of the benzyl radical, whose intensity peaked at 30 ps delay, was readily observed and is displayed in fig. 2. The same spectrum was also obtained following the IR MPD of toluene and benzylchloride in the presence of Ar_ The fluorescence lifetimes (extrapolated to zero pressure) of the A1 level (447.7 nm) and A2 level (446.5 nm) were 1 S i 0.2 and 1.6 + 0.3 p respectively, in good agreement with lifetimes obtained in a matrix [I9-211, and in the gas phase [22] _Our results differ from those obtained in the gas phase by Okamura et al. [ 151, and the reason for this discrepancy is not clear.
CO, laser beam that we probe is not uniform. This should not be important at low fluences, where the fragment yields increase very rapidly with laser energy, and therefore the signals arise mainly from the high intensity part of the CO, laser pulse. The changes in the average dissociation rate with fluence (fig_ 1) are too large to be caused by errors due to the inherent averaging of the experiment_ An important feature of the experiment is the short duration of the CO, laser pulse compared with the appearance times of the fragments produced at low laser fluences. Thus, the appearance times that we measure reflect the microscopic unimolecular reaction rates, rather than steady state rates (as derived by Quack [23], for example) which result from a convolution of the optical pumping rates and the reaction rates during the CO, laser pulse_ The latter are phenomenological values, specific to IR MPE, whereas the former are the true (albeit average) unimolecular reaction rates. It is therefore possible to relate our measured reaction rates to the average energy in the
4
BENZYLAMINE lo’-
: ”
4_ Discussion It is important to emphasize that the dissociation rates reported here are average values which derive from an unknown distribution of excess energies achieved via IR MPE. The width and shape of this distribution are unknown, but model calculations indicate that it is narrower than a thermal distribution [2] _The accuracy of our low-fluence measurements is insufficient to justify fitting the appearance times to a distribution of reaction rates, and thus only average values are derived. In addition, with our experimental geometry, the spatial intensity profile of the
IO5 40
50 EXCESS
60 ENERGY
70
80
! 0
(kc01 mol-‘)
Fig_ 3. Unimolecular reaction rate of BA versus energy in excess of the threshold energy, calculated according to ref. 1241 using Am, Earn. and vibrational frequencies from ref. [6]_ The brackets indicate the range that was observed in the IR hlPD of BA.
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dissociating BA molecules using thermal activation data [6]. The unimolecular reaction rate at energy E can be extracted from the thermal average as per Forst [24, 251, and the results of such a deconvolution are shown in fig. 3. We use the frequency factor A,, activation energy Ea_ and vibrational frequencies of BA from ref. [6], and we calculate densities of states according fo ref. [26]_ A similar curve is obtained using RRKM theory [25,26] with vibrational frequencies for the molecule and the activated comples from ref. (61 * _From this curve, we can estimate the average energy in the dissociating BA molecule [6], and hence the number of “excess” CO, laser photons absorbed. These estimates are summarized in table 1. and show that when the fluence changed from 10 to 30 J cm-2, the average number of excess photons absorbed increased from 17 lo 75. At low fluences, the excess energy is apparently fluence dependent. since most of the dissociation occurs after the termination of the CO, laser pulse. At higher fluences, however, it was shown that the energy above reaction threshold is intensity dependent. and is controlled by the competition between the net optical pumping rate and the reaction rdte [I ,2,28-301. From table 1 it is evident that the energy available for fragment excitation is rather high. Most of this energy will reside in the benzyl radical, since it has nwny more vibrational degrees of freedom than NH,. and thus, rile benzyl radical is produced vibrationally “hot”. This esplains why a proper LIF spectrum of the brnzyl radical is observed only at high buffer gas pressures and long delays_ when relaxation of the ndsccnf vibrarional excitation insures that a structured benzyl spscrrum is observed_
5. Conclusion The average energy above dissociation threshold, following the IR hlPE of benzylamine was derived from measurements of the appearance times of the Nl IL fragment at low CO, laser fluences. This energy depends on the CO, laser fluence, and at IO-30 J cm-‘, It varied from 50 to 70 kcal mol-l above disso* l-or 3 discussion of the convolution of thermal activation dsta see &o ref. 1371. 392
19 Au,mt 1983
ciation threshold. As expected, most of this ener7 resides in the benzyl radical, which is produced vibrationally “hot”. Measurements of “excess” energies via IR MPD are important in studies concerned with enerM disposal in fragments and their comparisons with statistical theories. Such studies are now possible for NH, from BA, since its spectrum, although complex, has been rather well analyzed_
Acknowledgement
We wish to thank G. Hancock for unpublished results on the LIF of NH,, and G. Smith for unpublished results on the lifetimes of the benzyl radical.
References [l] hi .N.R. Ashfold and G. Hancock, Chem. Sot. Spec. Res. Rep. 4 (1981) 73; P-A. Schulz, AaS. Sudbo, 1-J. Krajnovich, H.S. Kwok, Y-R. Shen and Y-T. Lee, Ann. Rev. Phys. Chem. 30 (1979) 379. [ 21 D.S. King, in: Dynamics of the excited state, ed. KP. Lawley (WiIey, New York. 1982) p_ 105 and references therein. [3 ] IL Reisler, F. Kong, A_hl. Renlund and C. Wit@, J. Chem. Phys. 76 (1982) 997. [4] H. ReisIer, F. Kens, C. Wittig, J. Stone, E. Thiele and hl.Iz. Goodman, 1. Chem. Phys. 77 (1982) 328. [5] S. Ruhman, 0. Anner and Y. Haas, J. Chem. Sot. Faraday Disc. 75 (1983) to be published. f6] D.M. Golden, R3i. SoBy. N_A_ Gac and S-W_ Benson, J. Am. Chem. Sot. 94 (1972) 363. [7] H. Reisler, M. hlangir and C. Wittig, J. Chem. Phys. 47 (1980) 49. [8] I. Welti, Infrared vapor spectra (Heiden, London, 1970). [9] I;. Dressier and D.A. Ramsay, Phil. Trans. Roy. Sot. A251 (1959) 553. [lo] 11. KroIl, J. Chem. Phys. 63 (1975) 319. [ 111 A-J. Roberts and G. Hancock, unpublished results. [12] G_ Porter and B. Ward, J. Chim. Phys. 61 (1964) 102. [13] C. Cossart-hlajos and S. Leach, J. Chem. Phys. 56 (1972) 1534;64 (1976) 4006. [141 D-hi. Brenner, G.P. Smith and R.N. Zare, J. Am. Chem. Sot. 98 (1976) 6706. 1151 T. Okamura, T.R. Charlton and B.A. Thrush, Chem. Phys. Letters 88 (1982) 369. 1161 H-H. Nelson and J-R. McDonald, J. Phys. Chem. 86 (1982) 1242. 1171 hl. Heaven, L. Dimauro and TA. hliller, Chem. Phys.
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1181 M-N-R. A&fold, G. Hancock and A-J. Roberts, Faraday Disc. C&em. Sot. 62 (1979) 204. [19] T. Okamnra and T-Tanaka, J_ Phys. Chem. 79 (1975) 2728. 1201 T. Okarnura, Ii. Obi and I. Tanaka, Chem. Phys. Letters 26 (1974) 218. [21] J-D. Laposa and V, Morrison, Chem. Phys. Letters 28 (1974) 270. 1221 Dhl. Brenner, G-P. Smith and R.N. Zare, unpublished results. [23] hf. Quack, in: Dynamics of the excited states, ed. K.P. Iawley (Tsirey, New York, 1982) p. 39.5. 1241 W. Forst, J, Phys. Chem. 76 (1972) 342.
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f25] W. Forst, Theory of urdmoleeular react (Academic Press, New York, 1973)_ 1261 PJ. Robinson and K.A. Holbrook, UnimolecuJarreactions Wiley, New York, 1972). [27] H. Reisler, F&T. Pessine, Y. Haas and C. Wittig, J. Chem Phys. 78 (1983) 378. [58] CM. Miller and R.N. Zare, Chem. Phys. Letters 71 (1980) 376. [29] A.M. Renlund, H. Reisler and C. Wttig, Chem. Phys. Letters 78 (1981) 40. 1301 X1.N.R. AsMold, G. Hancock and hi_ Hardrake, 3. Photothem. 14 (1980) 85,
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