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J. Phys. Chem. A 2010, 114, 12972–12978
Dynamics of Highly Excited Nitroaromatics Bingxing Wang,†,‡ Benkang Liu,†,‡ Yanqiu Wang,† and Li Wang*,† State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Dalian 116023, China, and Graduate School of the Chinese Academy of Sciences, Beijing 100049, China ReceiVed: May 23, 2010; ReVised Manuscript ReceiVed: October 21, 2010
Although the photodissociation of nitroaromatics in low excitation electronic states has been extensively studied in recent decades, little is known about the highly excited electronic states. The fragmentation dynamics of three nitroaromatics, nitrobenzene, o-nitrotoluene, and m-nitrotoluene, in highly excited states, populated by the absorption of two photons at 271 nm, are studied with time-of-flight mass spectrometry. The temporal evolutions of the highly excited states are monitored by one-photon ionization at 408 nm. The transients of parent and fragment ions exhibit two ultrafast deactivation processes. The first process is ultrafast internal conversion from the initial excitation to Rydberg states in tens of femtoseconds. The second one is conversion from the Rydberg states to the vibrational manifold in the ground electronic states within hundreds of femtoseconds. The internal conversion process is accelerated by methyl substitution. In o-nitrotoluene, the two processes become much faster due to the hydrogen transfer from the CH3 to the NO2 group (ortho effect). Introduction 1-11
Dynamics of molecules in highly excited states below or above the ionization potential have attracted much attention recently. Below ionization, these states are Rydberg states, while those above the lowest ionization thresholds are superexcited states (SE).1,2 These two highly excited states play important roles in the photophysical and photochemical processes of large molecules. For example, controlling photochemical reactions by using chirped lasers has been achieved experimentally;3,4 however, the mechanism has remained mysterious. Recent experiments5 revealed that coherence plays a minor role in the fragmentation of isolated molecules by shaped near-IR pulses. The femtosecond coherent-control photodissociation of pnitrotoluene could be well explained by the ladder-switching mechanism. The parent molecule is ionized first via multiphoton absorption (ladder climbing). The second step is photoisomerization, followed by fragmentation (ladder switching). Pulse shaping enhances the ladder switching processes, while transformlimited pulses enhance the ladder-climbing processes. VUV radiation, UV-vis multiphoton absorption, and electron impact have been applied for preparing highly excited states.1,2 Because of the short-lived nature of highly excited states, it is difficult to probe the dynamics with the spectroscopy method, such as absorption or fluorescence spectroscopy. Recently, intense femtosecond lasers have become a powerful tool for studying the time-resolved dynamics of highly excited states. For example, the time-resolved relaxation dynamics of phenol,1 aliphatic amines,2 ketones,6 naphthalene series,7 azulene,8 and acetylene9 in highly excited or superexcited (SE) states have been investigated recently. Phenol in the SE state1 exhibited a partial doubly excited character and decayed into vibrationally excited Rydberg states in tens of femtoseconds, which was followed by a picosecond process. Deactivation of the superexcited aliphatic amines2 consisted of two decay components, * Corresponding author,
[email protected]. † State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics. ‡ Graduate School of the Chinese Academy of Sciences.
corresponding to two lifetimes: 60-180 fs and 0.4-7.0 ps. The first process involved the excited wavepacket motion out of the Franck-Condon region and down to the lower lying Rydberg state from the near-ionization state. The second picoseconds process was the dissociation reaction from the lower-lying state. The relaxation dynamics of superexcited naphthalene and 1-aminonaphthalene4 showed two predominant relaxation processes in hundreds of femtoseconds and several picoseconds, respectively. The femtosecond relaxation was attributed to an ultrafast IC from the initially prepared states to Rydberg states, which was observed in both the parent and fragment transients. In highly excited ketones,6 the first bond cleavage proceeded in a manner similar to that in the S1 state but much faster. The dissociation of neutral parents in Rydberg state occurred in several picoseconds,6,7 which could only be observed in the fragment channels. In the superexcited azulene,8 ultrafast internal conversion from the S4 state to S2 was followed by internal conversion to S0. Doubly excited states above the IP could autoionize or undergo dissociative ionization to Rydberg states, whose origins lied below the IP. For the doubly excited state lying below the IP, which could be general in aromatic systems, only the last decay route occurred.8 Nitro-containing energetic materials have attracted a great deal attention in recent decades because of their important physical and chemical properties. Multiphoton ionization of these molecules by nanosecond and femtosecond lasers produced a few small fragments.5,10-26 By using femtosecond laser pulses,14-17 the dissociation process was overridden by a faster multiphoton ionization process, resulting in parent ions and heavy fragments, which was known as a ladder climbing mechanism. A few works on nitro-containing compounds and ultrafast femtosecond laser pulses have been reported.14-17 The primary fragments were attributed to the decomposition of parent ions by the ladder climbing mechanism. Recently, the potential application of femtosecond photoionization mass spectrometry has been demonstrated in the efficient detection of the energetic compounds,17 which could produce specific fragments and molecular ions. The nanosecond photodissociation dynamics of nitroaromatics18-23 have been extensively studied. Three primary
10.1021/jp104727p 2010 American Chemical Society Published on Web 11/23/2010
Dynamics of Highly Excited Nitroaromatics dissociation pathways of nitrobenzene between 220 and 280 nm have been detected with nanosecond laser pulses: NO2, NO, and O elimination.18 A nitro-nitrite isomerization of nitrobenzene was suggested to take place before fragmentation. With ultrafast electron diffraction,19 the C6H5O radical was observed, and the formation time was measured to be about 8.8 ps upon excitation at 266.7 nm. This radical, corresponding to the NO loss channel, was found to be dominant in 266.7 nm photodissociation. Excitation to electronic states higher than S1 that were produced by 266.7 nm resulted in an efficient internal IC, followed by ISC to a repulsive triplet state. Photodissociation of nitrobenzene at 193, 248, and 266 nm and o-nitrotoluene at 193 and 248 nm was investigated using multimass ion imaging methods.20 Both NO and NO2 elimination channels were observed. In o-nitrotoluene, there was an additional OH elimination. Photodissociation of nitrobenzene at 266 nm took place in about 60 ns. Direct fragmentation from nitrobenzene was assumed to play a major role rather than the isomerization mechanism. The photodissociation product OH of o-nitrotoluene21,22 was formed in about 60 and 120 ns with 193 and 248 nm excitation, respectively. The rearrangement process of nitrobenzene to phenylnitrite has been predicted by theoretical calculations.21,24-26 To date, few studies on the time-resolved dynamics of highly excited nitroaromatics have been reported. In this paper, the ultrafast dynamics of highly excited nitroaromatics were investigated with time-of-flight mass spectrometry. The time evolution of the optically prepared electronic state was monitored by recording the transients of the parent and fragment ions. The deactivation process of the high-lying excited states could be derived from the transients. The fragmentation and deactivation mechanism is discussed. Experimental Section Femtosecond time-resolved mass spectrometry27 has been described elsewhere in detail. Briefly, our femtosecond laser was a home-built, solid-state, chirped-pulse amplified Ti: sapphire femtosecond laser composed of a seed oscillator, an amplifier with a stretcher, and a compressor. The fundamental light was centered at 814 nm (with a 30 nm bandwidth, 70 fs pulse width, and 20 Hz repetition rate). After frequency doubling with a thin BBO crystal, the output was decomposed into two beams of 814 and 408 nm (the second harmonic, with a 6 nm bandwidth at 3.04 eV) by a dichroic beam splitter. The 271 nm (the third harmonic, with a roughly 3 nm bandwidth at 4.57 eV) laser beam was generated by sum frequency mixing of these two beams in another BBO crystal. These beams served as pump and probe lights, respectively. The probe beam was temporally delayed relative to the pump beam by a computer-controlled step-motor-driven linear translator (Sigma Koki, SGSP26-150). The two laser beams were focused by fused silica lenses with f ) 380 and 450 mm, respectively, and then collinearly introduced into the vacuum chamber with a dichroic mirror. Commercial nitroaromatics (nitrobenzene, o-nitrotoluene, and m-nitrotoluene, without further purification) were mixed with the carrier gas helium (0.15 MPa to vacuum). The samples were heated to about 373 K to obtain good signal-to-noise ratios. The seeded gas was expanded supersonically into the vacuum chamber through a pulsed valve (General Valve with a 0.5 mm orifice). The supersonic molecular beam was collimated by a conical skimmer and intersected the two laser beams perpendicularly. Ions created in the intersection region were extracted and accelerated by a Wiley-McLaren type time-of-flight mass spectrometer. After flying a 40 cm field-free distance, ions were
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Figure 1. Mass spectra of nitrobenzene recorded with pump light only (a) and probe light only (b) and the temporal overlap region of the pump and probe (c). The pump and probe beams were 271 and 408 nm, respectively.
detected by a two-stage microchannel plate (MCP) detector and recorded by a digital oscilloscope (Tektronix Inc., TDS3054B). Each TOF mass spectrum was the average result over 532 laser shots. The pump-probe transients corresponded to an average of over 12 scans of the delay line. The detection chamber was kept below 5 × 10-6 Pa with the molecular beam on. Under the same experimental conditions, 1% Xe in helium gas was used for measuring the cross-correlation function between the pump and probe and for calibrating the laser intensity. The cross-correlation function was determined by fitting the transient signal of the Xe ion. Following the intensity calibrating method,28 based upon the ponderomotive energy shift of photoelectrons of Xe, the intensities of the pump and probe varied in the ranges of 4 × 1010 to 1 × 1011 W/cm2 and 6 × 1011 to 2 × 1012 W/cm2, respectively. Results and Discussion Mass Spectra. Spectra a and b of Figure 1 showed typical mass spectra of nitrobenzene under pump and probe light irradiation, respectively. Figure 1c illustrated the mass spectrum in the pump-probe temporal overlap region. The laser intensities were carefully adjusted to produce above mass spectra. The ionization due to only the probe was negligible, as shown in Figure 1b. Parent ion and fragment ions, C6H5+, C5H5+, C4H3+, and NO+, could be produced under the pump light irradiation, as shown in Figure 1a. Nitrobenzene, whose ionization potential is 9.94 eV,29 could be ionized by the absorption of three photons of 271 nm (4.57 eV) with an excess energy of 3.75 eV, followed by fragmentation. The pump light intensity was carefully adjusted to minimize the parent ion signal, which means that the laser intensity roughly met the requirement for three-photon absorption but was strong enough for two-photon absorption. Multiphoton probe ionization of nitrobenzene at 408 nm required absorption of four photons, which was not satisfied in the current experiments. Figure 1a indicated that fragmentation of the
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nitrobenzene cation was completed in the laser pulse duration, which resulted in the uncertainty in the power dependence of the parent ion yields. In the 90 fs, 375 nm experiment,14 the laser power dependence of the nitrobenzene cation was 2.2 and was attributed to a three-photon absorption process. However, in recent experiments,17 the laser power dependences at 375 and 325 nm were similar. Absorption of three photons at 325 nm (11.4 eV) was sufficient to activate the ionic dissociation channels, while absorption of three photons at 375 nm (9.93 eV) was too low. The ionization mechanism was inconsistent with a direct three-photon process. An intermediate state was suggested to play a key role,17 followed by one or two photon ionization and further fragmentation. Multiphoton ionization at 408 nm was negligible in our experiments, as shown in Figure 1b. The electron-energy-loss spectrum30 and photoelectron spectrum31 experiments on nitrobenzene indicated that there was a continuous absorption band from 8.8 eV to the first ionization potential. This band was accessible by the absorption of two 271 nm photons. Theoretical calculations32,33 also found that there was a highly excited electronic state around the total energy of two photons at 271 nm, whose molecular orbital was characterized as the 5b1 state. A weak transition around 4.35 eV was assigned to be a mixture state of the 1B2u state and a charge transfer between the ring and the nitro group.30-33 In our experiment, this 1B2u state served as an intermediate state in the two 271 nm photon excitation process. The absorption spectrum of nitrobenzene10,30 indicated that there were some valence shell excitations into π* orbitals around 10.2, 10.9, 11.7, 12.8, and 13.8 eV. Most of these valence shell excitations could be achieved by the absorption of one photon of 271 or 408 nm from the prepopulated excited state. From these valence excitations, dissociation occurred, resulting in fragment ions. The mass spectrum of nitrobenzene obtained in the pumpprobe temporal overlapping region is shown in Figure 1c. The previous photodissociation of nitrobenzene and nitrotoluene18-21 showed that dissociation products were mainly in their ground electronic states. For example, ionization of the neutral dissociation product NO (IP ) 9.26 eV) from the highly excited parent required the absorption of three photons at 271 nm with a total energy of 13.71 eV, which was sufficient to ionize the neutral parent molecules. The appearance energy for the dissociation channel leading to NO2+ production was about 26 ( 1 eV,15 which was too high to be accessible in the current experiments. The ionization potential of NO2 was 9.586 eV. Clearly, these fragment ions were mainly due to the fragmentation of parent cations. Similar situations occurred in m-nitrotoluene and o-nitrotoluene, whose ionization potentials were 9.24 and 9.46 eV, respectively.29 In the cases of m-nitrotoluene and o-nitrotoluene, the pump and probe laser intensities were slightly different from that of nitrobenzene. The laser intensities of the pump and probe were carefully adjusted to minimize strong parent ions and excessive fragment ions by the one color laser beam. The mass spectra of these three nitroaromatics recorded in the temporal overlapping region are illustrated in Figure 2. The primary fragmentation of nitrobenzene was the NO2 loss channel, and the corresponding fragment peaks are dominant in Figure 2a, from the cleavage of the C-NO2 bond or C-ONO after the isomerization. Direct bond cleavage competed with the rearrangement in the dissociation of the nitrobenzene ion.27 A minor peak of C6H5O+ corresponding to the NO loss channel was observed in nitrobenzene, which was similar to photoionization at 375 nm.14
Wang et al.
Figure 2. Mass spectra of (a) nitrobenzene, (b) m-nitrotoluene, and (c) o-nitrotoluene recorded in the temporal overlap region of the pump and probe.
The NO loss channel was absent in m-nitrotoluene while the fragment ion C7H7O+ was preferred, as shown in Figure 2b, similar to the dissociations at 206 and 412 nm.35 A small amount of NO+ was observed. Among these mass spectra, NO2+ was not observed. The presence of NO+ and the absence of NO2+ were likely due to the isomerization of the parent ions, followed by fragmentation. The isomerization from C-NO2 to C-ONO has been predicted both in neutral parents18,20,24,25 and in parent ions.34 The NO elimination channel was a minor reaction.18 In o-nitrotoluene, a strong signal from C7H6NO+ was observed, which corresponded to the OH loss channel,11,22 as illustrated in Figure 2c. The geometry of nitrotoluene36 was heavily influenced by the relative position of the methyl group to the nitro group. In o-nitrotoluene, the methyl group was adjacent to the nitro group, and the perturbation was the maximum leading to the non-coplanarity of the nitro group with the aromatic ring. Hydrogen transfer from the methyl group to the NO2 group in o-nitrotoluene (the ortho effect) resulted in the OH loss channel. Among these three nitroaromatics, the shortest lifetime for o-nitrotoluene37 suggested large structural changes in the excited state. In the o-nitrotoluene cation, the OH loss channel, produced via hydrogen transfer from the methyl group to the NO2 group,14 had a lower barrier of 0.60 eV than that of isomerization.17 Due to this ortho effect, the highly excited o-nitrotoluene and the parent cation deactivated or dissociated much faster than nitrobenzene and m-nitrotoluene. The mass spectra also showed a few other fragment ions, C5H5+, C4H3+, and C3H3+, which were due to the further decomposition of the primary products. Transient Signals of Nitroaromatics. The temporal evolution of the highly excited nitroaromatics was monitored by the recording intensities of the parent ions and primary fragment ions in time-of-flight mass spectra. The time-dependent transient signals of dominant ions of nitrobenzene, m-nitrotoluene, and o-nitrotoluene were normalized and are illustrated in Figure 3-5, respectively. The cross-correlation function was also shown in these figures for determination of the zero delay time.
Dynamics of Highly Excited Nitroaromatics
Figure 3. Time-resolved transients of parent ions and fragment ions of nitrobenzne. The experimental data were normalized and are shown as empty circles. The solid lines are the fitting results. The short dashdot lines represent the cross-correlation functions.
In contrast to previous femtosecond experiments, both parent ions and fragment ions in our experiments exhibited ultrafast dynamics. The lifetimes of the lowest excited singlet state (S1) and the lowest triplet state (T1)13 were about 6 and 400-900 ps, respectively. The NO elimination of nitrobenzene in the S2 state19 involved rearrangement with bond breaking and bond forming to yield NO and phenoxyl radicals. These processes were the dominant dissociation pathways of the S2 excited state19 on the time scale of 8.8 ( 2.2 ps. Recent nanosecond photodissociation experiments20 of nitrobenzene and o-nitrotoluene at 193, 248 and 266 nm showed long-lived excited states with lifetimes of about 10 ns. The OH formation times for the 193 and 248 nm photolysis of o-nitrotoluene22 were about 60 and 120 ns, respectively. The lifetime of the triplet upon excitation at 366 nm in liquids38 was about 1 ns. These previous reports addressed the dissociation from excited Sn states20 (for example, S7 and S8 for nitrobenzene and o-nitrotoluene at a 193 nm excitation, respectively). These differences also implied the excited states addressed in our experiment were different from previously reported excited states.13,19-22,38 As mentioned above, fragment ions detected in our experiments were due to dissociation of the parent ions, because the probe laser was not intense enough for multiphoton ionization detection of neutral fragment species. Obviously, in our experiment, transient signals of the fragment ions of these nitroaromatics showed behavior similar to that of the parent ion transients, and no distinct buildup was observed. These experimental observations also indicated that fragment ions were formed from dissociations of ionized parents in our experiments. Furthermore, if these fragment ions were produced from the ionization of the dissociation species of the neutral parents, the time constants derived from various fragment ions should be different due to the different dissociation channels involved.2 The two-step decay model40 has been successfully applied to describe the deactivation process of the highly excited aliphatic amines2 and ketones.6 The same model was applied
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Figure 4. Time-resolved transients of parent ions and fragment ions of m-nitrotoluene. The experimental data were normalized and are shown as empty circles. The solid lines are the fitting results. The short dash-dot lines represent the cross-correlation functions.
to fit the observed ion signals of the nitroaromatics, as indicated in eq 1. This two-step model described a deactivation process of the initially excited state I to a low-lying excited state II on the τ1 time scale, followed by a subsequent decay to product state III on the τ2 time scale. The time-dependent populations of state I and state II could be expressed as eqs 2 and 3. τ1
τ2
I 98 II 98 III
(1)
SΙ ) A1 exp(-t/τ1) + c3
(2)
SΙΙ ) A2 [exp(-t/τ2) - exp(-t/τ1)]
(3)
The multiphoton ionization of state I generated the parent ion signal; therefore, the transient profile of the parent ions of these three nitroaromatics could be expressed by the convolution of formula 2 with the cross-correlation function. The observed fragment ions came from the dissociation of ionic parents, generated from both state I and II through the multiphoton ionization due to the probe light. The fragment ion transient SF described the fragmentation of the ionized parent from the state I and deactivation dynamics of states I and II. The corresponding time-resolved transient signals were described by a linear combination of SI and SII, the convolution result of eq 4 with the cross-correlation function.
SF ) SΙ + SΙΙ ) A2 exp(-t/τ1) + A3 exp(-t/τ2) + c3 (4) where Ai is the amplitude associated with the pump and probe intensity and molecular density and τ1 and τ2 are time constants.
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Wang et al. TABLE 1: Fitting Results of the Transients of Three Nitroaromatics in Figures 3-5 probed species nitrobenzene
parent (τp) parent (τn) C6H5+ C4H3+ NO+ m-nitrotoluene parent (τp) parent (τn) C7H7+ C3H3+ NO+ o-nitrotoluene parent (τp) parent (τn) C7H6NO+ C7H7+ C5H5+
Figure 5. Time-resolved transients of parent ions and fragment ions of o-nitrotoluene. The experimental data were normalized and are shown as empty circles. The solid lines are the fitting results. The short dashdot lines represent the cross-correlation functions.
As illustrated in Figure 3a, Figure 4a, and Figure 5a, there are weak enhancements in the negative pump-probe delay time. These enhanced signals were due to excitation by 408 nm. Two possible channels were responsible to these nonzero components. The first one was four 408 nm photon ionization, followed by one 271 nm photon excitation, which resulted in the depopulation of the ground state. This channel decreased the parent ion signal, produced by four 408 nm photon ionization. Due to the nature of multiphoton ionization, the parent ion intensity was proportional to I4 (I represented 408 nm laser intensity). Even for the m-nitrotoluene, which had the biggest enhancement in the negative delay range among these three nitroaromatics, the dependence of the parent ion intensity on the 408 nm laser intensity was unconspicuous. Furthermore, the parent ion intensity in the positive delay range was independent of the 271 nm intensity. Considering the fragment ions’ intensity and transient profiles in the negative pump-probe delay time, ionization of these nitroaromatics by 408 nm were relative weak, and further fragmentations were negligible. Another possible channel was three 408 nm photon excitation, followed by one 271 nm photon ionization. A transient profile of the parent ions of these three nitroaromatics in the negative time delay region could be expressed by the convolution of formula 2 with the cross-correlation function. The time constants deduced from the parent ions and fragment ions of nitrobenzene, m-nitrotoluene, and o-nitrotoluene are marked in Figures 3-5, respectively. The fitting parameters are listed in Table 1. τp and τn in Figure 3-5 and Table 1 represent the decay time in the positive and negative time delay regions, respectively. Depopulation rates (τn) of these excitations are quite similar to those (τp) by 271 nm in the positive time delay region. These phenomena implied that same electronic states were populated by three 408 nm photon excitation and two 271 nm photon excitation. Absorption of three 408 nm photons populated the nitroaromatics into the same electronic states, 5b1, followed by one 271 nm photon ionization, which resulted in enhancements
τ1 (fs)
A1
71 ( 1 50 ( 1 67 ( 1 67 ( 1 57 ( 1 55 ( 1 50 ( 1 52 ( 1 63 ( 1 64 ( 1 19 ( 1 22 ( 1 16 ( 1 15 ( 1 12 ( 1
0.38 0.45 0.36 0.21 0.4 0.60 0.43 0.31 0.22 0.27 0.74 0.71 0.71 0.63 0.62
τ2 (fs)
A2
213 ( 13 0.04 320 ( 13 0.04 279 ( 13 0.05
c3
0.01 0.016 0.003
320 ( 13 0.03 -0.003 344 ( 13 0.05 0.024 375 ( 13 0.05 0.017 187 ( 13 0.04 -0.004 228 ( 13 0.04 0.01 156 ( 13 0.06 0.013
in the negative time delay region. Due to the absorption efficiency differences between 271 and 408 nm, the highly excited state was much more efficiently populated by absorption of two 271 nm photons than by absorption of three 408 nm photons. Furthermore, excitation by absorption of three 408 nm photons occurred through a virtual state, while excitation by absorption of two 271 nm photons occurred via a real state, 1 B2u. The transients of the different fragments in our experiments exhibited nearly identical decay time constants, which were similar to observations of aliphatic amines in their superexcited states.2 The dynamics of naphthalene and 1-animonaphthalene7 also indicated transients of fragment ions included fragmentation dynamics of the ionized parents. C6H5NO+ ions were not observed in our experiments, similar to the observations in nanosecond photodissociation dynamics34 and the threshold photoelectron-photoion coincidence experiment41 of nitrobenzene molecular ion. Clearly, the direct bond cleavage channel to C6H5+ competed with the rearrangement processes to NO+ and C6H5O+. In photolysis experiments of nitrobenzene molecular ions, the rate constants for the direct bond cleavage to C6H5+ increased rapidly with the internal energy, from microseconds41 to nanoseconds34 while the photolysis laser wavelength changed from 607.5 to 488 nm. The direct bond cleavage channel was dominant in the photodissociation of nitrobenzene ions at 514.5 and 488.0 nm.34 Considering the similarity of the transients of the parent ions and fragment ions and the fitness of the decay model, the possible deactivation mechanism of highly excited nitroaromatics is illustrated in Figure 6. The energies of electronic states and ionization potentials were based upon the reported values for nitrobenzene.10,30,32 Appearance potentials of fragment ions of nitrobenzene are also illustrated in Figure 6. Although the energy levels were slightly different for these three nitroaromatics, the electronic sketches were similar. The high-lying excited state near the ionization potential was efficiently populated via an intermediate state by the pump laser. In nitrobenzene, the intermediate state was S3 with 1B2u symmetry,12,20,30,32 which had been interpreted as a localized benzene-type π-π* excitation.12 Due to the double electron excitation nature, the initial populated state in the 5b1(1π*, π*NO) molecular orbital was unstable. Below the ionization threshold, there were Rydberg series that converge to ionization limits. It is reasonable to suppose the initial wavepackets moved out of the Franck-Condon region to these Rydberg states via ultrafast internal conversion in tens of femtoseconds (τ1). Subsequently,
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J. Phys. Chem. A, Vol. 114, No. 50, 2010 12977 Fitting results clearly indicated that decay rates of both the parent ions and fragment ions in o-nitrotoluene became much faster than the other two nitroaromatics. The time needed for hydrogen atom intramolecular transfer in o-nitrotoluene was shorter than 60 fs.14 Hydrogen transfer from the methyl group to the NO2 group in o-nitrotoluene (the ortho effect) enhanced the decay rate of the excited o-nitrotoluene. The fragment ions C7H6NO+, corresponding to OH loss channel, and NO+, observed in the mass spectrum in Figure 2c, indicated that the fragmentation occurred after nitro-nitrite isomerization. The subsequent fragmentations of C7H7+ in m-nitrotoluene and C6H5+ in nitrobenzene were similar, as shown in Figure 3d and Figure 4d. Conclusion
Figure 6. Possible deactivation mechanism of superexcited nitroaromatics. All the electronic states and ionization potentials were of nitrobenzene. Thin solid lines represent ionization potentials, while dashed lines represent valence excitations.
the deactivation of Rydberg states to highly excited vibrational levels of the ground electronic states in hundreds of femtoseconds (τ2) resulted in the subsequent rearrangement and fragmentation. These neutral fragments, however, could not be ionized by probe light due to the huge geometry change. The second decay components were not observed in the parent transients. The fragment ions could be produced from the valence excited parent ions, generated through the ionization of both the initial excited state and the Rydberg states. In naphthalene and 1-aminonaphthalene,7 this phenomenon was explained by the complete dissociation of the neutral parent. The time constants derived from the transients of different fragment ions in our experiments were similar. The absence of the long-lived components in the parent transients might be due to the complete dissociative ionization of the neutral parent by the probe beam, as suggested in the superexcited aliphatic amines.2 Similar phenomena have also been found in other molecules, such as benzene,42,43 ketones,6 and 1,3-cyclohexadiene.44 Methyl substitution accelerated the deactivation process of the S1 state of aromatics.39 Comparing the transients of nitrobenzene and m-nitrotoluene in Figure 3a and Figure 4a, methyl substitution indeed slightly accelerated the decay rate of the initial excited state from 68 ( 1 fs for nitrobenzene to 55 ( 1 fs for m-nitrotoluene. For the direct C-N bond cleavage transients in Figure 3b and Figure 4b, the methyl substitution, however, slowed down the dissociation rate, from 213 ( 13 fs for C6H5+ in nitrobenzene to 320 ( 13 fs for C7H7+ in m-nitrotoluene. The NO loss channel in m-nitrotoluene was also slower than in nitrobenzene, corresponding to lifetimes from 279 ( 13 fs in nitrobenzene to 375 ( 13 fs in m-nitrotoluene, as illustrated in Figure 3c and Figure 4c. These two opposite effects of methyl substitution on the dynamics of excited states have been reported recently.39,45 Methyl substitution of benzenes39 led to an even faster second π-π* decay rate. The methyl substitution of R,β-enones45 had only small effects on the S2 decay times, while the position of methyl substitution affected the relaxation rate from the S1 surface and dynamics of the branching ratios to the products. As illustrated in Figure 5 a-c, the time constants of o-nitrotoluene in the first step were much smaller than the laser pulse width. Although the time constants obtained were less credible, it was still informative for comparing these constants with those of other nitroaromatics.
Nitroaromatics in highly excited states near ionization potentials were investigated using femtosecond time-resolved mass spectrometry. The molecules were populated into a highly excited state near IP by the absorption of two photons at 271 nm. The time evolutions of the optically prepared wavepackets were monitored by temporal delayed 408 nm photoionization. A two-step decay model was applied to explain the ultrafast dynamics of the observed transients of the parent ions and fragment ions. The initial wavepackets left the Franck-Condon region and entered the Rydberg states by IC within tens of femtoseconds. Relaxation from the Rydberg states to the highly excited vibrational levels of the ground electronic states took hundreds of femtoseconds. The transients of the parent ions, produced from the initial excited states, reflected the decay dynamics of the first step. The fragment ions were produced from the dissociation of parent ions in the first step and fragmentation of ionized Rydberg states. From the transients of fragment ions, the second step decayed in hundreds of femtoseconds. In o-nitrotoluene, the two deactivation processes were faster than those in nitrobenzene and m-nitrotoluene. While methyl substitution sped up the first step decay rate and slowed down the second step, the nature of the reaction was still unclear. Further investigations, including time-resolve photoelectron imaging experiments and accurate theoretical calculations, such as the time-dependent wavepacket approach, are necessary to understand the physical nature of highly excited states of nitroaromatics. Acknowledgment. This work is supported by the National Natural Science Foundation of China (Grant No. 20633070). References and Notes (1) Schick, C. P.; Weber, P. M. J. Phys. Chem. A 2001, 105, 3725– 3734. (2) Solling, T. I.; Kotting, C.; Zewail, A. H. J. Phys. Chem. A 2003, 107, 10872–10887. (3) Pastirk, I.; Brown, E. J.; Zhang, Q. G.; Dantus, M. J. Chem. Phys. 1998, 108, 4375–4378. (4) Brixner, T.; Gerber, G. ChemPhysChem 2003, 4, 418–438. (5) Lozovoy, V. V.; Zhu, X.; Gunaratne, T. C.; Harris, D. A.; Shane, J. C.; Dantus, M. J. Phys. Chem. A 2008, 112, 3789–3812. (6) Solling, T. I.; Diau, E. W. G.; Kotting, C.; Feyter, S. D.; Zewail, A. H. ChemPhysChem 2002, 3, 79–97. (7) Montero, R.; Castano, F.; Martinez, R.; Longarte, A. J. Phys. Chem. A 2009, 113, 952–958. (8) Blanchet, V.; Raffael, K.; Turri, G.; Chatel, B.; Girard, B.; Garcia, I. A.; Wilkinson, I.; Whitaker, B. J. J. Chem. Phys. 2008, 128, 164318. (9) Zamith, S.; Blanchet, V.; Girard, B.; Andersson, J.; Sorensen, S. L.; Hjelte, I.; Bjorneholm, O.; Gauyacq, D.; Norin, J.; Mauritsson, J.; L’Huillier, A. J. Chem. Phys. 2003, 119, 3763–3773. (10) Cooper, L.; Shpinkova, L. G.; Rennie, E. E.; Holland, D. M. P.; Shaw, D. A. Int. J. Mass Spectrom. 2001, 207, 223–239.
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