ArF Laser-induced decomposition of simple energetic niolecules

Volume

107. number

CHEMICAL

6

ArF LASER-INDUCED Anita M. RENLUND

DECOMPOSlTlON

12 hluch

15 June

LETTERS

OF SIMPLE ENERGETIC

MOLECULES

1981

*

and Wayne M. TROTT

Sarrdin National Laboratories. Albuquerque. Received

PHYSICS

New Mexico 87175, USA

1984

Chemiluminesccnt products formed benzene have been studied. Implications

from

ArF laser-induced

to esplosive

chemistry

decomposition are briefly

of nitromrthane, discussed.

rr-prop!!

nitrate

and nirro-

(C,H,N@) and n-propyl nitrate (C;H,ONO,). The ArF laser was chosen because all three molecules absorb strongly at 193 nm. and recent work on ArF laser dissociation of nitromethane [ I,?] provides a good comparison for the present esperiments. The three molecules contain functional groups representative of most HEs: nirroalkanes, nitroaromatics and nitrate esters. These experiments may help elucidate mechanisms of laser initiation of esplosives and also address the possible role of the electronically escited state in shock initiation. which remains poorly understod. Owens and Sharma [6]. for esample. have found that dissociation of certain esplosive molecules in a sub-threshold shock more closely resembles the dissociation patterns observed in W photolysis than in thermal decomposition. Delpuech and co-workers [7,8] have also sugested that production of electronically excited species may be required in shock initiation of explosives. In the present esperinients the eszct relationship between the laser pulse inrensity and the pulse energy is complicated, but increases in the fluence result in increases in intensity. The fluence is the more easily measured parameter. and we report our results here as fluence-dependent data. We have monitored the chemiluminescent products as a function of laser fluence to identify pathways which may be important at both fast and slow rates of ener,T deposition.

1. Introduction In order to investigate chemical reactions which may be important in initiation of explosive materials, we have recently begun studies on laser-induced decomposition of simple energetic molecules in the gas phase. Clearly, the conditions in the gas phase are far removed from the density and temperature of detonating HEs; nevertheless, the rapid excitation possible with high-power lasers provides a comparable energy deposition rate which is more easily controlled_ These experiments may therefore provide insight into reaction energetics and indicate important classes of reactions to investigate in condensed phase HEs. Recently, several papers have appeared in the literature on laser-induced decomposition of gas-phase energetic molecules [l-S]. The aim of these studies has been to elucidate the initial photodissociation pathways by monitoring photofragments. These reactions have been studied both in molecular beams and il bulk systems and have relied on techniques such as time-of-flight mass spectromstry [ 1,2], chemiluminescsnce [2,4], laser-excited fluorescence [3] and transic,gt absorption spectroscopy [S] to detect product species. In this letter, we describe experiments where ckemiluminescent products have been monitored following ArF laser excitation of three simple energetic molecules: nitromethane (CH,NO,), nitrobenzene * This work was performed a~ Sandia National Laboratories. AIbuquerque. NM. supported by the US Department of Energy

under

Contract

No. DE-ACO476DP00789.

0 009-2614/84/S 03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B-V.

555

Volume 107. number 6

CHEhIlCAL PHYSICS LETTERS

2. Experimental In these experiments sample vapor at low pressure was dissociated in a fluorescence cell by the unfocused output from an ArF laser. Chemiluminescence was viewed perpendicular to the laser both through narrow bandpass filters with a photomultiplier tube (PMT) and by a spectrometer/optical multichannel analyzer (OMA) combination. The unfocused output from the ArF laser (Lumonits TE-262-2) was apertured prior to entering the cell to minimize scattered light. The cell was fitted with suprasil windows which were attached to short baffle arms to minimize the signal due to window fluorescence; the total path length of the cell was 32.5 cm. Tine iaser-beam cross section was 1.6 X 0.6 cm at the center of the cell, and the laser energy was monitored both before and after passing through the cell. The laser fluence was usually varied by taking advantage of the usual decrease in laser output during operation using a given gas fill and occasionally by inserting a 50% partially transmitting mirror in front of the cell. The data were independent of the method chosen for changing the laser fluence. For the experiments reported here, the fluence was varied over the range lo-125 mJ cm-‘. The typical pulse duration was 15 ns fwhm. For time-resolved measurements, emission from the center of the cell was imaged withf/l optics onto the PMT (EMI 9659QB). Narrow bandpass filters (typically IO nm fwhm) were used to isolate spectral regions of interest. The signals from the PMT were digitized (Biomation 4500, 10 ns minimum gate width) and averaged over 16 laser firings for each data trace. Chemiluminescence spectra were obtained by focusing the emitted light onto the slit of a $ m spectrom eter (PAR). The dispersed spectrum was viewed by a gatable, intensified vidicon detector (PAR 1254) which was fitted with a UV scintillator to give effective detection over the range 200-800 nm. Blocking filters were used when appropriate to eliminate interference from second-order W emission. The signals were processed by the PAR OMAR computer system, and signals from 16 laser ftigs were summed to obtain a spectrum. The detector was gated on for 2 MS; the Laser was fired in the middle of the gate pulse. The spectra therefore result from the time-integrated

emission during the first ~1 ps after the laser was 556

ISJune

fired. The wavelength response of the spectrometer/ detector was calibrated using a standard quartz-halogen lamp and all spectra shown here have been corrected accordingly. Nitromethane, n-propyl nitrate (both Aldrich) and nitrobenzene (Fluka AC) samples were generally degassed at 77 K prior to use. Gas chromatography and JR spectral analysis of the sample vapors showed no measurable concentration of impurities, and no further purification was attempted. For aU measurements. the sample was flowed through the cell to avoid buildup of photoproducts. The flow was regulated with a needle valve and the pressures were measured with a capacitance manometer.

3. Results At the high laser fluences employed in this study, we expect the photodissociation transition R-NO,

+ hu + R + NO2

(1)

to be partially saturated by the incident radiation field. ln this case, the Beer-Lambert description of photon absorption is not appropriate. Instead, a bleaching wave analysis is necessary and the I-D radiation transport equation must be solved:

6m, 0 + 1 wx, 0 6x

c

6t

aqx

t)

t)p(x 7

=

SP(Xvf)

, t

(3)

‘-

where I is the laser intensity, p is the density of absorbing species, u is the photodissociation cross section, and c is the velocity of light in the medium. This can be solved analytically [9] to give the energy El that passes a distance I through a sample: El = Es In 11 + ex~--o~OO[ex~~O/Es)

-

1 II,

(3)

where po is the initial sample density, E,-, is the energy incident on the sample, and the saturation energy, Es = hula This solution assumes a temporally rectangular laser pulse and negligible photon absorption via processes other than the initial photodissociation. The results of this analysis for a 32.5 cm path length and 120 mJ incident energy are shown in fig. 1 for two different photodissociation cross sections, 2 X lo-l7 cm2 and 3 X lo-l7 cm* (1.7 X lo-l7 cm2 is the measured value for nitromethane [l] ). Also

CHEMICAL PHYSICS LETTERS

Volume 107, number 6

IsJune

1981

r

I

a 1

0.1 0

zo

I

40

60 SWRE

I

I

I

eo

IO0

IZO

H,

.-

PRESSURE I mrorr1

Fig. I. Absorption of 193 MI radiation as a function of reactant pressure. Solid lines are linear least-squares tits to esperimental results. Dashed lines ore results from bleaching--\iave (B\V) calculations.

2-

u

.50

LPC

Ylavelenglh shown

in fig. I are the laser absorption data for nitromethane, tr-propyl nitrate and nitrobenzene at different pressures. The measured absorption does not resemble a bleaching wavefunction, even though the calculated sat;ration energy for u = 2 X 10-i’ cm’ is50mJcm _ Instead, the experimental results are consistent with significant absorption of the Ia.ser energy by the fragments of the initial photod~soc~tion process. This will be discussed further below for the case of nitromethane. In order to calculate the laser fluence at the center of the absorption cell (which is the volume of interaction viewed by the OMA and PMT), we have used the measured “effective” absorp tion cross sections obtained from the data in fip. 1.

The spectra of the visible emission obtained following ArF-laser photolysis of 20 mTorr nitromethane, n-propyl nitrate and nitrobenzene

2a, 2b and 2c, respectively.

are shown in figs.

The laser fluences

in these

‘10

OCC

(nm)

Fig 2. Spectra of visible chrmilumintsccncc (corrected for detection system response) produced sit photolysis of (3) ~lrontelh~e, (b) n-propyl nitrare. and fc) nitrolxnzzne. Laser flurnce xts 125 mJ cm-* ior each spectrum. were 125 mJ cm-3 _A blocking filter effectiveiy eliminated all light below 400 nm. In separate experiments. NO(A-X) y-bands were readily observed in the W from high-fluence photolysis of nitromethane and n-propyl nitrate. Because of the low pressures of these experiments most of the molecules will not undergo a collision during the observation esperiments

time (
species were made from the wavelength

positions

of 557

CHEhUCAL

Volume 107. number 6

the emissions, and, where possible, by comparing the measured radiative lifetimes with published values. Emission through a 430 run narrowband filter following photolysis of 1 mTorr nitromethane displayed a characteristic lifetime in reasonable agreement with the known CH(A-X) lifetime of 534 nm [IO]. Lifetimes of the emission at 630 and at 760 nm, again from photolysis of low-pressure nitromethane, were measured to be 20.8 and 27.0 /.Is, respectively, which fall within the range of radiative lifetimes which have been reported for NO,(A 2B7) [ 1 I]. In addition. we measured the NO,(A ?B2) emission lifetime as a function of sample pressure. From these experiments,

PHYSICS

LETTERS

JSJune1984

Table 1 Fluence dependencefor production products. Iemission a fl P

Molecule

NO2 nitromethane n-propyl nitrate nitrobenzene a) Saturates b, Increases

of chemiluminescent

(A)

1.1 =) 1.0 a) 1.0-4.0

at high fluence. with increasing

b)

CWN

NO(A)

C,(d)

2.3 3.6 3.7

1.4 1.2 -

5.3

fluence.

we calculate rate coefficients of 1.4 X lo-” and 2.4 X 10-‘” cm3 molecule-1 s-’ for quenching by nitromethane and ,z-propyl nitrate (and their photofragments), respectively. Donnelly et al. 1121, have found similar fast rates of quenching of NO,(A) with several collision partners. ln figs. 3a and 3b we show the dependence of the emission intensity of the chemiluminescent products on the ArF laser intensity following the photolysis

---4---_ NO-

.

.

.

.

._-- *- ,*-.

_--.

___._,___mli

of nitromethane and nitrobenzene. Results for )Ipropyl nitrate are similar to those for nitromethane. The emission intensity of each species was obtained from the integrated areas under the peaks of the spectra and/or from the intensity of the time-resolved emission viewed through narrowband filters. The emission intensities could generally be fit to the laser fluence, @, raised to some power, p: Ia @, ami the values of p obtained are shown in table 1.

4. Discussion 4.1. Nitromethane

Fig. 3. Emission intensity of chemihtminescent products from the photolysis of (a) nitromethane and (b) nitrobenzene as a function of laser fluence. The units of intensity are arbitrary for each of the emitting species and do not reflect relative intensitier Solid lines are least-squares fits to the data. in the case of nitromethane. the dashed curve through the data for NOa shows results from BW calculations assurning a primary photodissociation cross section near that 18 ported in ref. [ 1 ] and a constant branching ratio between NOI and NOI as a function of laser fluence.

558

Excitation of nitromethane at 193 run occurs via a n-n* transition localized primarily on the NO2 moiety [13]. Recently, Blais [l] and Butler et al. [2], have studied the primary photodissociation in molecular beam experiments. These studies have shown that the photodissociation process (SIread calculated using vahres from ref. [14] ) CH,NO,

+ CH, + NO,,

AIImd

= 63 kcal mol-‘, (4)

has a near-unity

quantum

yield with a cross section of

Volume 107. number 6

CHEMICAL PHYSICS LETTERS

1.7 X IO-l7 cm’. and that nascent NO? is formed both in the ground state and in the A ‘Bz electronically excited stare. It was also concluded the the CH, fragment is born with little internal excitation. There is sufficient energy in a single 193 nm photon (148 kcal mol-‘) to dissociate nitromethane and to excite NO,(A) to its dissociation limit producing NO(X) + 0 as nascent products. The partial saturation of this primary photodissociation process at high laser fiuences is evident in our chemiluminescence data for NO*(A). As shown in fig. 3a, the NO*(A) emission intensity initially increases linearly with laser fluence but begins to level off at higher fluences. This behavior is consistent with the dashed curve in fig. 3a which shows the calculated (normalized) yield of NO,(A) as a function of laser fluence assuming a cross section and quantum yield for the primary photodissociation process near that reported in ref. [ 1 ] and a constant branching ratio. This yield, AP/PO, is given by the relation Ap

-= PO

evWo/Es) - 1 ev&, IQ ’

(5)

where po, EO and Es are as defined above. As mentioned previously, the discrepancy between the measured absorption and that predicted by the bleaching wave analysis may be explained by substantial absorption of the laser output by photofragments. When we assume only single-photon excitation of the CH, and NO2 fragments and an average photodissociation cross section of 8 X lo-l8 cm’, we obtain good agreement with the absorption data shown in fig. 1. These sequential absorptions and additional multiplephoton processes apparently give rise to the other chemiluminescent products observed in these experiments. While NO(X) can be formed in a one-photon step, NO(A) formation is endothermic. The formation of NO(A) with a power dependence of 1.4 indicates that this is a multiple-photon process. It seems most likely that the formation of NO(A) occurs as result of a sequential process; highly vibrationally or electronically excited NO, is excited by a second photon to give the observed NO(A-X) r-bands. That the fluence dependence is not more closely equal to 2 is likely due to saturation of the absorption transition of the nitromethane molecule at high fluences. The formation of CH(A) as a direct photofragment is also consistent with a sequential absorption process where the CH,

fragment

1.5 June 1984

is further dissociated:

CH, + CH(A) + H,.

AH,,,

= 171 kca! mol-‘.

(6)

4.2. n-propyl nitrate As in the case of nitromethane. escitation of tzpropyl nitrate at 193 run likely involves a rr-rr* transition localized primarily on the NO, [ 151. Again. the principal chemiluminescent species observed are CH(A), W(A) and NO,(A). The main differences in the results lie in the relative yields of CH(A) and NO,(A) as seen in figs. 3a and 3b. There is a smaller yield of chemiluminescent products. including N$(A), from n-propyl nitrate than from nitromethane photolysis. Since the R-NO, bond in IIpropyl nitrate is weaker than in nitromethane, it ap pears that either less total energy is partirioned to the NO? fragment in the dissociation process or that the energy is preferentially deposited in channels other than electronic escitation. The fluence dependences for the formation of the fragments, however. are very similar for nitromethane and rl-propyl nitrate. Again. because of reaction energetics, NO,(A) seems to be a product of a sin&-photon absorpt;on process: C,H,ONO,

--f C,H,O

+ NO?.

A/I react = 42 kcal mol-‘.

(7)

Also. the NO, yield levels off at higher fluences. consistent with bleaching of this primary photodissociation process. The shape of the NO-, emissions in the two spectra are very similar. Direc; formation of NO(X) in the first dissociation step is probable, and th formation of NO(A) may occur in a fashion similar to that suggested for nitromerhane photolysis. Escitation by additional photons of the C,H,O fragment is required to yield CH(A). 4.2 ffitroberzene The 193 run absorption band in nitrobenzene has been described as a superposition of two bands, the W. + W1 and Wu + W5 transition: ;~d]. The former transition has the character of local excitation within the nitro group while the latter is localized primarily on the benzene ring and shows vibrational structure apparently corresponding to the breathing mode of the ring. While the chemiluminescence spectra of 559

Volume 107, number 6

CHEMICAL PHYSICS LETTERS

nitromethane and n-propyl nitrate appear quite similar, the spectrum obtained from ArF laser photolysis of nitrobenzene is markedly different (see fig. 2~). C?(d-a) Swan bands are very prominent and little emission from NO?(A) is observed. Also, there is little emission in the UV due to NO(A). of particular note is the behavior of the NO,(A) yield as a function of laser fluence. At low fluences, it varies linearly with fluence and shows no energy threshold. At higher fluences, however, instead of showing the saturation effect like nitromethane and ~propyl nitrate (fig. 3) it begins to increase more sharply with fluence. Yet even under high Guence conditions such as those used to obtain the spectrum in fig. 2c, comparatively little NO?(A) is formed. At low fluences where only single-photon absorption would be significant, the only Iikely dissociation channel energetic enough to yield NOZ(A) is the

simple bond scission reaction: C,H,NO,

+ CsHg + NO,,

QH ze;let = 7 1 kcal mol-’ .

(8)

At higher laser fluences, and thus higher photon intensities, channels leading to other dissociation products become energetically allowed. These are likely to occur by true multiple photon excitation of nitrobenzene or nitrobenzene*, as this seems the most reasonable explanation for the apparent fluence dependence of the NO,(A) formation. It may be that the unimolecular dissociation rate of nitrobenzene’ with one photon absorbed is slow enough to allow competition from the laser excitation process. It is interesting here that the yield of NOI can be accelerated by increasing the rate of energy deposition suggesting that the primary photodi~o~~tion pathway (or branching ratios among more than one reactic: pathway) may change as a function of laser intensity. These results may address the relative sensitivity of some HEs under various methods of initiation. The initial steps which lead to detonation of HEs are not well understood. In slow thermal excitation, it is assumed that the weakest bonds, usually those binding a NO2 group to the molecule, will break first. Several theories for shock initiation exist, from extensive fragmentation of the molecule [ 171 to essentially thermal mode~g based on hot-spot formation f l&19]_ It may be that decomposition promoted by relatively 560

15June1984

slow means (thermal) follows a different reaction channel from that which is promoted by rapid excitation (e.g. shocks). Further work is in progress in this laboratory to monitor products in the very early stages of reactions of condensed-phase HEs. as well as on IR laser photolysis of the molecules studied here [20]. Acknowledgement We gratefully acknowledge the technical of H.C. Richardson and J-C. Pabst.

assistance

References 111 N.C. Blais. J. Chcm. Phys 79 (1983) 1723.

I21 LJ. Butler, D. Ktajnovich

Y.T. Lee, G. Ondrey and R. Bersohn, J. Chem. Phys. 79 (1983) 1708. 131 P.E. Schoen, M.J. Marrone, J.M. Schnur and LS. Goldberg, Chem. Phys. Letters 90 (1982) 272. 141 KG. Spears and S.P. Brugge. Chem. Phys. Letters 54 (1978) 373. 151 C. Cappellos, J. Photochem. 17 (1981) 213. 161 F.J. Owens and J. Sharma, J. Appt. Phys. 51 (1980) 1494. 171 A. Delpuech and J. Cherxille. Propellants Explor 3 (1978) 169;4 (1979) 61, 121. ISI A. Deipuech, J. Cherville and C. hlichaud, in: Proceedings of the 7th Symposium on Detonation, Anapolis, hfaryland (1981) p. 36. J. Appl. Phys. 34 (1963) I91 L.h% Frantz and J.S. Nod-. 23461. 1~01 J. Brzozoaski, P. Bunker. N. Elander and P. Erman, Astrophys. J. 207 (1976) 414. 1111 V.hl. Donnelly and F. Kaufman, J. Chem. Phys. 66 (1977) 4100; 69 (1978) 1456. [121 V.hl. Donnelly, D.G. Iieil and F. Kaufman, J. Chem. Phyr 71 (1979) 659. r131 S. Nagakura, hlol. Phys. 3 (1960) 152. I141 D.R. Stull and ii. Prophet_ JANAF the~o~~erni~ tabIes 37.2nd M., NSRDSNBS (1971); H. Okabe, Photochemistry of small molecules (\VileyInterscience, New York_ 1978); S.W. Benson, Thermochemical kinetics, 2nd Ed. (Wiley, New York, 1976). ItsI V.M. Csizmadia, S.A. Houfden, G.J. Koves, J.hL Boggs and LG. Csizmadia, J. Org. Chem. 33 (1973) 2281. 1161 S. Nagakura, M. Kojima and Y. Marnyama, J. hlol Spectxy. 13 0964) 174. (171 F-E. Walker and R.J. Wasley. Propellants Explor 1 (1976) 73. I’S1 kW. Campbell, W-C Davis and J.R.. Ttavis. Phys. Fiuids 4 (1961) 498. 1191A.W. Campbell, W.C. Davis. J.B. Ramsay and J.R Travis, Phyf Fiuids4 (1961) 511. PO1 W-M. Trott and AM. Renlund, paper TUMS. Conference on Lasers and Electra-optics, Baltimore, hlaryland (1983).