Photodissociation of carboxylic acids: dynamics of OH formation

Journal of Photochemistry and Photobiology C: Photochemistry Reviews 3 (2003) 165–182

Review

Photodissociation of carboxylic acids: dynamics of OH formation Prakash D. Naik, Hari P. Upadhyaya, Awadhesh Kumar, Avinash V. Sapre, Jai P. Mittal∗ Radiation Chemistry and Chemical Dynamics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India

Abstract In this review article, recent studies on the photodissociation dynamics of carboxylic acids carried out in our laboratory are presented. The dynamics are investigated by mapping the energy partitioning in the nascent photoproduct OH using laser-induced fluorescence spectroscopy. To understand the effect of the nature of the C–C bond on the dissociation dynamics, both saturated (acetic) as well as unsaturated (acrylic and propiolic) carboxylic acids are investigated. In all of the carboxylic acids studied, a high percentage of the available energy is partitioned into the product translational state, indicating the presence of an exit barrier in the dissociative potential energy surface. Based on the energy partitioning, the quantum yield and the OH formation rate, the photoexcitation dynamics of carboxylic acids are revealed. © 2002 Japanese Photochemistry Association. Published by Elsevier Science B.V. All rights reserved. Keywords: Photodissociation dynamics; Laser-induced fluorescence; Quantum yield; Absorption cross-section; OH spectroscopy; Nascent state distribution; Carboxylic acids

Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1. General introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2. Electronic spectroscopy of OH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Material and methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Laser photolysis–laser-induced fluorescence (LP–LIF) set-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. UV-visible fluorescence studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Absorption cross-section measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Laser photolysis–laser-induced fluorescence (LP–LIF) studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Rotational state distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Vibrational state distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Spin–orbit state distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5. Population of the -doublets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6. Translational energy in products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7. OH growth at 193 nm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.8. Visible fluorescence from excited acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.8.1. Fluorescence from 193-nm excitation of acrylic acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.8.2. Fluorescence from 193-nm excitation of propiolic acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.9. Absorption cross-section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.10. Quantum yield of OH formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Photodissociation dynamics of acetic acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.1. Dominant photochemical path to OH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.2. Barrier in the dissociation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.3. Models of the dissociation dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ∗ Corresponding author. Also associated with: Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore, India. Fax: +91-22-5505151/5519331. E-mail addresses: [email protected] (P.D. Naik), [email protected] (J.P. Mittal).

1389-5567/02/$ 20.00 © 2002 Japanese Photochemistry Association. Published by Elsevier Science B.V. All rights reserved. PII: S 1 3 8 9 - 5 5 6 7 ( 0 2 ) 0 0 0 3 7 - 0

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4.2. Photodissociation dynamics of acrylic acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1. Nature of the dissociative state leading to OH formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2. Existence of an exit channel barrier in the OH-forming reaction . . . . . . . . . . . . . . . . . . . . . . . 4.2.3. Assignment of fluorescence for 193-nm excitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.3.1. Fluorescence (275–305 and 290–345 nm range) with λmax at 290 and 318 nm . . 4.2.3.2. Fluorescence (380–500 nm range) with λmax at 438 nm . . . . . . . . . . . . . . . . . . . . . . . 4.3. Photodissociation dynamics of propiolic acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1. Energy partitioning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.2. Dissociative state of propiolic acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction 1.1. General introduction Upon photoexcitation, molecules undergo several photochemical processes. Several experimental and theoretical studies have been devoted to molecular dissociation process, as this is the initiation of many chemical reactions in the atmosphere. An excited state, which undergoes dissociation due to its short lifetime, usually exhibits a broadband structure in the absorption. A broad absorption spectrum gives little information about the character of the excited state. Information about the excited state of the parent molecule, i.e. the geometrical structure of the excited state, and the photodissociation dynamics can be obtained from the fragments formed in the dissociation process. State-resolved measurements, probing the population of individual states of the photofragment, provide microscopic details about the chemical reaction. Studies on the partitioning of the available energy among different states of a photofragment often point to the mechanism of a reaction. A non-statistical distribution of the energy partitioning indicates the direct nature of the reaction, where reaction occurs on a time-scale shorter than the rotational period of the molecule. On the other hand, a statistical distribution points out the complex nature of a dissociation mechanism, with a dissociation lifetime longer than the rotational period of the molecule. The photodissociation of carboxylic acids is important due to its role in atmospheric, combustion and interstellar chemistry. These acids are end-products or important intermediates in the oxidation of hydrocarbons. Some carboxylic acids, such as formic acid, have been identified, and some, such as propiolic acid, are expected to be present in interstellar space [1,2]. Carboxylic acids exist as a mixture of monomer and dimer in the gas phase, and this is due to the formation of strong hydrogen-bonded dimers. There are few studies on the UV spectroscopy of carboxylic acids outside of the spectra of formic and acetic acids reported by Barnes and Simpson [3]. The monomer and dimer absorption cross-sections have been measured by Singleton et al. [4] at ∼265 nm, the peak maximum of the n–␲∗ transition. Recently, Hintze et al. [5], have estimated the photoabsorp-

177 177 178 178 178 178 179 179 179 180 180

tion cross-sections for the monomers and dimers of acetic, propionic, butyric and isobutyric acids from 195 to 200 nm. In the thermal decomposition of organic acids, the major channels are either dehydration or decarboxylation reactions [6]. Ab initio calculations [7] predict a concerted four-centre transition state for both the dehydration and decarboxylation reactions, with threshold energies of 76.0 and 77.3 kcal mol−1 , respectively. The UV laser-induced photodissociation of carboxylic acids has been investigated by several workers in recent years [8–16], primarily because it generates the atmospherically important OH radical. The OH can be produced either directly in a primary step by the C–O bond cleavage or in a secondary reaction via dissociation of HOCO, the primary product formed after the C–C bond cleavage. The dynamics of dissociation leading to the formation of OH can distinguish between these two mechanisms. In the reverse reaction with respect to C–O bond cleavage producing OH, a barrier is observed in some carboxylic acids [13,14]. Several workers have studied the photodissociation of carboxylic acids and have explored the dissociation dynamics by mapping the energy partitioning into the product states. Recently, we have studied the photodissociation dynamics of acetic, acrylic and propiolic acids [8,15,16]. The photodissociation dynamics of 1 (n, ␲∗ ) excited acetic acid is interesting in several aspects and has been studied in some detail [11,13,17,18]. The photodissociation may occur by multiple reaction channels, arising from the cleavage of either the weaker C–C single bond or the stronger C–O single bond. There are two channels leading to the product OH [19]: CH3 COOH → CH3 CO + OH, H ◦ = 110.3 kcal mol−1 (1) CH3 COOH → CH3 + OCOH,

H ◦ = 86.4 kcal mol−1 (2a)

OCOH → CO + OH,

H ◦ = 34.1 kcal mol−1

(2b)

Guest and co-workers have investigated the photodissociation dynamics of acetic acid excited to the 1 (n, ␲∗ ) state at

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218 nm [17] and have shown that the acetic acid undergoes a solely two-body dissociation (reaction (1)) to produce OH. Such high selectivity in the OH reaction channel is very interesting and is maintained even at higher energies corresponding to 200-nm light [13]. The authors found that a very small fraction of the available energy goes into the internal energy of the OH fragment, and a large fraction appears in the translational energy. Based on the fact that a large fraction of the available energy is channelled into the translational mode and fact that the amount of energy in this mode is almost independent of the available energy, they predicted the existence of a 13 kcal mol−1 barrier for the dissociation process [13,17]. Recent ultrafast absorption studies of Owrutsky and Baronavski [11] support the existence of a barrier in reaction (1). In acrylic acid, the smallest ␣,␤-unsaturated carboxylic acid, the conjugation between two different types of double bonds, namely –C=C–, and –C=O, leads to interesting spectroscopy and photochemistry. There are only a few studies available on the photochemistry of acrylic acid. Most of the work related to acrylic acid is focused on polymerisation, and only a few studies are devoted to its molecular photochemistry. The following processes have been proposed as primary dissociation pathways: H2 C=CHCOOH + hν → H2 C=CH + HOCO

(3)

H2 C=CHCOOH + hν → H2 C=CHCO + OH

(4)

H2 C=CHCOOH + hν → H2 C=CH2 + CO2

(5)

H2 C=CHCOOH + hν → H2 C=C(H)OH + CO

(6)

Reactions (3) and (4) involve the cleavage of C–C and C–O single bonds, respectively, while reactions (5) and (6) are the decarboxylation and decarbonylation channels. Rosenfeld and Weiner [20] studied the photodissociation dynamics of acrylic acid at 248 and 193 nm using the infrared fluorescence technique and concluded that decarboxylation is the major primary pathway at both wavelengths. Based on the mass fragmentation spectral data, Miyoshi et al. [21] have pointed out the importance of reactions (4) and (6), in addition to the C–C bond cleavage (reaction (3)), in the photolysis at 193 nm. Recently, Kitchen et al. [14] have carried out an intensive study of the photodissociation dynamics of acrylic acid at 193 nm in a crossed laser-molecular beam apparatus by measuring the photofragment velocity distributions. They concluded that the cleavage of C–C and C–OH bonds is the major channel and decarboxylation the minor channel of dissociation. They confirmed the formation of the HOCO radical in the excited state. In a recent flash photolysis study on acrylic acid, Osborne et al. [22] measured the relative yields of some of the major products such as HOCO, CO2 and CO by the infrared diode laser absorption technique. They suggested that the reactions (3) and (4) and a combination of (5) and (6) constitute three competing channels, which lead to the formation of HOCO, CO2 and CO, respectively.

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Apart from experimental studies on the photodissociation of acrylic acid, a few theoretical studies on the ground-state surface are also available [12,23]. In recent studies, Fang [24] and Fang and Liu [25] have mapped the potential energy surfaces of various excited states of acrylic acid using complete active space self-consistent field (CASSCF) methods. They characterised three excited state surfaces and investigated the dissociation dynamics occurring on them. Propiolic acid is important from the atmospheric and combustion chemistry points of view, apart from its use in stereospecific synthesis [26]. As it is expected to be present in interstellar space, its microwave spectroscopy has been investigated in detail [1,2,27]. Theoretical calculations [2,28] predict the molecule to have planar geometry, with the hydroxyl proton adopting cis- (syn) or trans- (anti) conformations with respect to the carboxyl group, the syn conformer being more stable by 3.6 kcal mol−1 , with a rotational barrier of ∼6.4 kcal mol−1 . These calculations showed that its dipole moment originates almost entirely from the charge distribution of the COOH group, implying that the degree of conjugation between the acetylenic and carbonyl groups is rather modest. In the present article, we summarise the photodissociation dynamics studies on carboxylic acids (acetic, acrylic and propiolic) carried out by probing the energy partitioning in the different states of the product OH. As the work is based on the laser-induced fluorescence detection of OH formation, the electronic spectroscopy of OH is described in brief for the benefit of general readers. 1.2. Electronic spectroscopy of OH In a diatomic molecule like OH, the motion of the electron takes place in a reduced symmetry of the field, unlike in an atomic system, where the electron moves in a spherically symmetrical field. In this case, there is only symmetry of field about the internuclear axis, due to the electrostatic field of the two nuclei. The precession of the orbital angular momentum vector about this field, which is directed along the internuclear axis, gives the component ML (h/2π), where ML has values from −L to +L. Due to the electrostatic nature of the field, the states with the same ML values but different sign are degenerate. These different values of ML are represented by Λ, which is given by Λ = |ML |

(7)

Thus for a given value of L, there are L + 1 values for Λ, representing L + 1 distinct states with different energies. According to the international nomenclature, Λ = 0, 1, 2, 3, . . . are designated as , , , , . . . states. As in atoms, the spins of the individual electrons in the molecule give the resultant spin S. In the case of the Λ = 0,  state, the resultant spin is oriented in a fixed space, as long as the molecule does not rotate. In other cases (Λ = 0) the

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internal magnetic field in the internal nuclear axis resultant from the orbital angular motion causes the precession of S about it. It is designated as , and quantum theory allows 2S + 1 different values for it. The algebraic addition of Λ, the orbital angular momentum along the internuclear axis, and , the electronic spin angular momentum along the internuclear axis, gives the total electronic angular momentum about the internuclear axis. This total electronic angular momentum is designated by , where  = |Λ + |

(8)

The electronic term symbol of the diatomic species is designated as 2S+1 Λ . According to this nomenclature, the ground electronic state of OH is 2  and the first excited electronic state is 2 . The interaction of electron spin and orbital angular momentum splits each rotation state with Λ = 0 into two spin–orbit states. Thus, for the ground state of OH, the two spin–orbit states are 2 1/2 and 2 3/2 . At high rotational quantum number, the rotational angular momentum interacts strongly with the electronic angular momentum and lifts the degeneracy of the Λ = 0 states, which are doubly degenerate and gives the two components Λ+ (A ) and Λ− (A

).

2. Material and methods 2.1. Laser photolysis–laser-induced fluorescence (LP–LIF) set-up The schematic of the set-up used in the studies is shown in Fig. 1. The photolysis laser employed is an excimer laser (Lambda Physik Model Compex-102, fluorine version). The probe beam is the second harmonic output of a dye laser (Quantel, TDL90) pumped by the second harmonic (532 nm) of a seeded Nd:YAG laser (Quantel, YG980 E-20). The dye laser operated with DCM special dye with the fundamental wavelength tuning range of 600–640 nm. The photolysis and the probe laser beams traverse orthogonally through two pairs of windows to intersect at the centre of the reaction chamber, which is made of stainless steel and equipped with five arms and three ports for the pressure transducer, gas inlet and the vacuum pump. Two orthogonal pairs of arms are provided with MgF2 and quartz windows at the Brewster angle to facilitate transmission with reduced scattering of the photolysis and the probe beams, respectively. The fifth bottom arm is used to collect the fluorescence. The fluorescence was collected by 38-mm diameter lens of focal length 50 mm and detected by a photomultiplier tube (PMT, Hamamatsu, model R 928P). A band-pass filter (λcentre = 310 nm, FWHM = ±10 nm, T310 nm = 10%) was placed

Fig. 1. Schematic diagram of the LP–LIF experimental set-up.

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between the collecting lens and the PMT to cut-off the scattering from the photolysis laser. The fluorescence signal was gate-integrated by a boxcar (SRS 250), averaged for 30 or 100 laser shots and fed into an interface (SRS 245) for A/D conversion. A Pentium II PC was used to control the scan of the dye laser via an RS232 interface and to collect data through a GPIB interface using a control and data acquisition program. To correct for the laser intensity fluctuations, both the pump and the probe lasers were monitored by photodiodes and the fluorescence intensities were normalised. The laser frequency was calibrated using an optogalvanic cell (Fe–Ne) with an accuracy of ±0.3 cm−1 . The spectral resolution of the probe laser is 0.06 cm−1 . Acid vapour was flowed through the reaction chamber at a flow velocity of approximately 10 cm s−1 . The acid pressure was maintained at less than 100 mTorr, which was measured using a capacitance gauge (Pfeiffer Vacuum). The OH fragment was probed state-selectively by exciting the A2  ← X2 (0, 0) transition of OH (306–309 nm) and monitoring the subsequent A → X fluorescence. Both the laser beams were unfocused and attenuated to prevent saturation, and the LIF signal was confirmed to be linearly proportional to the laser power. Further, as a main branch and the adjacent satellite branch lines probe the population of the same state, the ratio of their LIF signals should be equal to the ratio of their Einstein coefficients if there is no saturation in the LIF signals. The Einstein coefficients for the main branches are around 10 times higher than their satellite branches. Hence, the saturation in LIF intensities was estimated from the ratio of their intensities and found to be negligible. The LIF spectra were measured at pump-probe delays of 80 and 200 ns and found to be identical, thereby ensuring that the collisional relaxation was negligible. The LIF signals were measured as a function of the acid pressure and found to be linear with the monomer acid pressure used. This result assures that the OH production from the photolysis of acid dimer was negligible. Acids (>99.5% purity) were used as supplied after several freeze–pump–thaw cycles. During irradiation, the windows of the photolysis laser developed a thick coating from the deposits of some photoproducts and were cleared regularly during the experiment to avoid the attenuation of the laser energy. 2.2. UV-visible fluorescence studies To obtain the fluorescence, if any, on photo-excitation, the LIF set-up was modified. One of the extended arms of the probe laser was removed, and a collection lens was placed at about 50 mm from the photolysis beam. The fluorescence was dispersed with a monochromator (Jarrell Ash, model 82-410; resolution = 3 nm) and detected with PMT. The signal was averaged for 300 shots at each wavelength and stored in a digital oscilloscope (LeCroy, 9350A) for further processing. In addition to the disperse fluorescence, the temporal profiles of the fluorescence at the peak maxima were measured to estimate the lifetime of the emitting species.

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2.3. Absorption cross-section measurement The absorption cell (50 cm long) was constructed from a 40-mm o.d. Pyrex tube and has MgF2 windows at each end. The sample enters the cell at one end of the cell and is pumped at the other end by a rotary pump. The sample pressure in the cell was measured with a capacitance manometer. The laser light was attenuated heavily by suitable substrate window and was collimated by the iris. The very low-intensity laser beam (<1 ␮J) was split into two parts: one was passed through the sample cell (signal) and the other used as a reference. The intensities of the signal and reference beams were measured by photodiodes (Becker and Heckl, PDM-400). The intensities of the signal beam without the sample in the cell and the reference beam were kept almost the same. The multiplication factors between these two beams were obtained before and after each set of readings. The intensities were measured with and without the sample in the cell to obtain the I0 /I ratio. 3. Results 3.1. Laser photolysis–laser-induced fluorescence (LP–LIF) studies The OH radical was produced by 193 and 248 nm photolysis of acrylic acid and by 193-nm photolysis of acetic and propiolic acids, and was probed by LIF. The A2 + (v = 0) ← X2 (v

= 0) transition of OH was used for the LIF measurements. Typical LIF spectra, with appropriate assignments [29], are shown in Figs. 2 and 3, respectively, for the (0, 0) and the (1, 1) bands of the A–X system of OH. The P, Q, and R branches refer to rotational transitions with N = −1, 0, and 1, respectively. The transitions represented by subscripts 1 and 21 originate from 3/2 , whereas those represented by 2 and 12 originate from 1/2 spin–orbit states. According to the parity selection rule (+ ↔ −), the Q branch arises from the − (A

) state, while the P and R branches originate from the + (A ) states. Relative populations of the OH fragment were determined by normalising the respective peak area of the rotational lines with respect to pump and probe laser intensities, pressure change, if any, and the respective Einstein absorption coefficients [30]. Spin–orbit and the Λ doublet ratios were calculated from the relative population of different states. The translational energy associated with the OH fragment was calculated from the Doppler profiles of the rotational lines. The detailed results are presented in the following sections. 3.2. Rotational state distribution The nascent rotational state population of OH radicals generated on photodissociation of the acids was used to construct a Boltzmann plot for obtaining the rotational temperature of nascent OH fragments. The rotational tem-

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Fig. 2. A portion of LIF spectrum of the A–X(0, 0) system of OH after photodissociation of acrylic acid (40 mTorr) for 193-nm photodissociation (pump-probe delay: ≈250 ns).

OH fragments are formed in the v

= 1 state. Although weak compared to the (0, 0) transition, a measurable LIF signal was obtained for the (1, 1) transition in the photodissociation of propiolic acid. From the Boltzmann fit to the rotational population of OH(v

= 1), it was found that OH carries an average rotational temperature of 620 ± 30 K. Assuming a Boltzmann distribution, a vibrational temperature can be obtained from the ratio of the LIF intensities of the same rotational line of the (0, 0) and (1, 1) transitions by
 I(R1,2 (N

); v

= 1) hc = exp −εv , (9) I(R1,2 (N

); v

= 0) kTV Fig. 3. A portion of the LIF spectrum of the A–X(1, 1) system of OH after photodissociation of propiolic acid (25 mTorr) for 193-nm photodissociation (pump-probe delay: ≈100 ns).

where εv is the energy difference between v

= 1 and v

= 0 for a given spin–orbit state and N

, and TV is the vibrational temperature. The constants h, c and k have

peratures obtained by the best fit to all of the points were 450 ± 50 and 360 ± 50 K, respectively, for 193 and 248 nm photodissociation for acrylic acids. For photodissociation of acetic acid and propiolic acid at 193 nm, the OH fragments are formed in the ground vibrational state, with rotational temperatures of 790 ± 80 and 800 ± 30 K, respectively. The typical Boltzmann plot is shown in Fig. 4 for acrylic acid photodissociation at 248 nm. 3.3. Vibrational state distribution The OH(1, 1) transition was scanned to determine the population of the OH fragment in v

= 1. An LIF signal was not observed in the case of acrylic and acetic acid photodissociation. Based on the experimental detection limit and the Frank–Condon factors relative to the OH(0, 0) transition, it was estimated that less than 5% of the total

Fig. 4. A Boltzmann plot of rotational distribution of the nascent OH radical generated on 248-nm photolysis of acrylic acid. The solid line is fit to the data points and represents a temperature of 360 ± 40 K.

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their usual meanings. The intensities of a few rotational lines P1 (1), P1 (2), Q1 (5) and Q1 (6) were used to estimate TV , and the average value for TV was calculated to be 1030 ± 80 K. 3.4. Spin–orbit state distribution The spin–orbit ratio, generally known as the F1 /F2 ratio, which gives the relative population in (3/2 ) and (1/2 ) states, was obtained from the ratio of population P1 (N)/P2 (N), Q1 (N)/Q2 (N), . . . , etc. For F1 (N) and F2 (N), since the J values are not the same, the population was normalised with a statistical weighting factor, i.e. 2J + 1. In the case of acetic acid, it was found that there is no preferential population of either state. However, in the photolysis of acrylic and propiolic acids, the 3/2 state is preferentially populated. In acrylic acid photolysis, the formation of the 3/2 state was more predominant at 193 nm compared to that for 248-nm photodissociation. Vasudev et al. [31] have explained the preferential population of 3/2 in terms of coupling between the initially prepared excited states with the nearby triplet state. The F1 (3/2 )/F2 (1/2 ) ratios are plotted versus the OH rotational quantum number in Fig. 5 for acrylic acid photolysis. 3.5. Population of the Λ-doublets The Λ-doublets arise from the orientation of the ␲-lobes of OH with respect to the plane of rotation. In the + (A ) state, the ␲-lobe lies in the plane of rotation, while in the − (A

) state, the ␲-lobe is perpendicular to the plane of rotation. The Λ doublet ratio [− (A

)/+ (A )] was obtained from Q1 (N)/P1 (N) or Q1 (N)/R1 (N). For Q1 (N) and P1 (N) or R1 (N), since the J values are the same, normalisation is not required. The relative populations of the Λ-doublets provide information about the exit channel dynamics during

Fig. 5. The statistically weighted spin–orbit and Λ-doublet ratios as a function of the rotational quantum number. The circles represent the Λ-doublet ratio and the squares represent the spin–orbit ratios. The filled circles and squares are for 193-nm photolysis, and the open one is for 248-nm photolysis of acrylic acid.

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the breaking of the chemical bond. In the case of acetic acid and propiolic acid photolysis, the ratio was one, within experimental error. It is surprising to see that in the photodissociation of acrylic acid, both for 193 and 248 nm photodissociation, the Λ-doublet ratio deviates from the statistical value of unity (∼0.8). In Fig. 5, the Λ-doublet ratio is plotted versus the rotational quantum number N for acrylic acid photolysis. 3.6. Translational energy in products Doppler profiles reflect the distribution, f(vz ), of the velocity component vz of the absorbing species along the direction of propagation of the probe laser beam via the linear Doppler shift ν = ν − ν0 = vz ν0 /c. For an isotropic velocity distribution, f(vz ) = f(vx ) = f(vy ), the average translational energy in the laboratory frame is given by ETlab (OH) = (3/2)[mOH v2z OH ], where v2z OH represents the second moment of the laboratory velocity distribution of the OH radical, which is represented by the equation  +∞ v2z f(vz ) dvz v2z OH = =c

−∞  +∞  (ν 2 −∞

− ν0 ) ν0

2 g(ν) dν.

(10)

In Eq. (10), c is the speed of light and g(ν) is the normalised Doppler profile of the OH radical. In the present case, g(ν) represents a Gaussian function. The linewidth and shape of the Doppler-broadened LIF line include contributions from the fragment molecular velocity, the thermal motion of the parent molecule and the finite probe laser linewidth. The width of the laser spectral profile of the probe laser beam was obtained from the OH Doppler profile measured in a thermalised condition. The FWHM of the spectral profile of the probe laser beam was estimated to be 0.07 cm−1 . The Doppler profile of the P1 (2) line for acrylic acid photolysis is shown in Fig. 6. De-convolution of the peak profiles with the spectral profile of the laser beam gives Doppler widths of 0.34 ± 0.04 and 0.29 ± 0.04 cm−1 , respectively, for the 193 and 248 nm photolysis of acrylic acid. More than 40 rotational line profiles were evaluated to estimate the average kinetic energies in the laboratory frame, ETlab (OH). The product translational energy in the centre-of-mass frame was obtained from ETlab (OH) and was found to be 13.2 ± 3.1 and 10.2 ± 2.1 kcal mol−1 for 193 and 248 nm photolysis, respectively, for acrylic acid. Similar analysis of Doppler profiles from 193-nm photolysis of acetic and propiolic acids yielded a translational energy of +5.0 10.0−2.8 and 24.4 ± 3.0 kcal mol−1 , respectively. 3.7. OH growth at 193 nm Temporal evolution of the product OH(ν

= 0) was obtained by varying the delay time between the photolysis and probe lasers. For 248-nm excitation of acrylic acid

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The growth rate of OH formation was computed by fitting the time-dependent data to an exponential function with correction for diffusion I(t) = A[1 − exp(−kt)] − kd t,

(11)

where k is the growth rate and kd is the linear diffusion rate at which OH is moving away from the observation zone due to its intrinsic velocity. The value of kd was estimated in the experiment with 248-nm photodissociation, in which OH formation was very fast. The rate constants for OH formation were estimated to be (1.6 ± 0.7) × 106 and ∼2 × 106 s−1 , respectively, for 193-nm photolysis of acrylic and propiolic acids. 3.8. Visible fluorescence from excited acids Fig. 6. Doppler profile of the P1 (2) line in the spectrum for (A) 193 nm and (B) 248 nm photodissociation of acrylic acid. The solid line drawn through the data points represents a Gaussian fit. The dotted line (C) represents the laser spectral profile (FWHM = 0.07 cm−1 ).

and 193-nm photolysis of acetic acid, the OH radicals were formed instantaneously during the laser pulse, while for 193-nm excitation of acrylic and propiolic acids, rise times of ∼2 and 0.5 ␮s, respectively, were observed. Fig. 7 shows the LIF intensity of the Q1 (3) line of OH versus the pump-probe delay time for 193-nm photolysis of acrylic acid. The first data point was taken when the probe laser was fired 500 ns before the pump laser, and subsequent measurements were carried out at 50-ns intervals. Each data point was the average of 100 pulses. Evolution of several other intense rotational lines, mainly the strong lines of the Q1 and P1 branches of the (0, 0) transition, was also investigated. All of the rotational lines studied showed similar growth times.

Intense fluorescence was observed for 193-nm laser excitation of acrylic and propiolic acids in the UV-Vis region, but there was no fluorescence for 193-nm excitation of acetic acid and 248-nm excitation of acrylic acid. These observations will be rationalised in detail in the following sections. 3.8.1. Fluorescence from 193-nm excitation of acrylic acid The fluorescence spectrum of excited acrylic acid is given in Fig. 8, which shows three distinct Gaussian peaks centred at 288, 318 and 438 nm. The peak at 318 nm is very strong and sharp, while other peaks are weak and somewhat broadened. The temporal profiles of the fluorescence were taken at the above peaks in order to obtain the lifetime of the emitting species. A typical decay profile with a single-exponential decay fit is shown in Fig. 9. The decay rate constants of fluorescence, estimated at the three observed peaks, are the same, within experimental error. The average value of the decay rate constant is (2.8 ± 0.7) × 106 s−1 .

Fig. 7. LIF intensity of OH generated in the dissociation of acrylic acid as a function of the pump-probe delay at (A) 193 nm and (B) 248 nm. The acrylic acid pressure is 15 mTorr.

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three distinct peaks, which cannot arise from the lowest excited state of the acrylic acid molecule formed from the photodissociation of its dimer. This leaves the only possibility of the initially excited parent molecule being the emitter.

Fig. 8. Fluorescence spectra obtained on exciting acrylic acid with a 193-nm laser pulse.

The emitter of the observed fluorescence may be either the excited parent molecule or an excited product. The plot of fluorescence signal versus laser intensity is found to be linear (up to 1.5 mJ cm−2 ) at the three observed peaks, suggesting the occurrence of a mono-photonic process. However, from the mono-photonic excitation, it is not possible energetically to obtain electronically excited fragments that can give the observed fluorescence. Hence, the observed fluorescence is assigned to the excited parent molecule. The fluorescence from the excited parent molecule has been reported for acetic acid and acetone in jet-cooled experiments [17,32]. In both studies, the observed fluorescence was attributed to the emission from the electronically excited parent molecule formed from the photodissociation of the parent dimer. However, for acrylic acid, the contribution from the dimer is ruled out on the basis of (1) the linear dependence of the fluorescence intensity with the monomer pressure (concentration), (2) the dimer concentration being very low in the pressure range studied (<3%), and (3) the

3.8.2. Fluorescence from 193-nm excitation of propiolic acid The dispersed fluorescence spectrum of excited propiolic acid at 193-nm (shown in Fig. 10) at 150-ns delay has three distinct peaks, at about 314, 440 and 490 nm, with a small peak at ∼285 nm. The fluorescence intensity at each peak wavelength was measured at almost the highest energy of the photolysis laser up to which linear dependence of the intensity with the energy was observed. The decay of fluorescence at peak wavelengths 314 and 440 nm is a single exponential; that at 490 nm is a better fit to a double exponential function. The dependence of fluorescence decay on the pressure of propiolic acid was studied to measure the quenching rate coefficients and the natural lifetimes of the emitting states at three peak wavelengths; these values are reported in Table 1. Fig. 11 shows a typical temporal profile of fluorescence at 440 nm after irradiation of propiolic acid (175 mTorr) at 193 nm with a single exponential fit of its decay. The plot in the inset depicts the linear dependence of the fluorescence decay rates on various pressures of Table 1 Measured lifetimes and quenching rate coefficients of the fluorescence emitting states after propiolic acid irradiation at 193 nm Wavelength (nm)

Lifetimes (ns)

Quenching rate coefficient (cm3 molecule−1 s−1 )

314 440

330 ± 25 430 ± 30

(9.2 ± 0.2) × 10−10 (2.8 ± 0.2) × 10−10

490

120 ± 20 1400 ± 100

(1.6 ± 0.1) × 10−10 (2.5 ± 0.2) × 10−10

Fig. 9. Temporal profile of fluorescence of acrylic acid on excitation at 193 nm for λmax = 318 nm.

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Fig. 10. The figure depicts the dispersed fluorescence spectrum for a 150-ns delay with three distinct peaks at about 314, 440 and 490 nm, with a small peak at ∼285 nm on photoexcitation of propiolic acid at 193 nm.

propiolic acid. Using the slope and the intercept of the linear plot, the quenching rate coefficient and the natural lifetime, respectively, were calculated. The quenching of fluorescence by propiolic acid is quite efficient, as inferred from the observed high quenching rate coefficients (Table 1). No information on the electronic states of propiolic acid is available either experimentally or theoretically. Hence,

the assignment of fluorescence cannot be made conclusively. The observed fluorescence between 250 and 450 nm is similar (with respect to the position of λmax ) to that of acrylic acid [33] after irradiation at 193 nm. Based on recent ab initio theoretical calculations [24,25], the fluorescence observed [29] from acrylic acid with λmax of 285 and 314 nm is attributed to the S2 → S0 transition, and that with λmax

Fig. 11. A typical observed fluorescence temporal profile (experimental points shown as open circles) at 440 nm after irradiation of PA (175 mTorr) at 193 nm with a single exponential fit (solid line, decay rate = 3.9 m s−1 ). The plot in the inset depicts the linear dependence of the fluorescence decay rates on varied pressures of propiolic acid.

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at 440 nm due to the S2 → S1 or the S2 → T1 transition. Based on the fluorescence assignment of acrylic acid and our theoretical calculations on propiolic acid, fluorescence in the region of 250–450 nm can be assigned to originate from the initially populated state, S2 of propiolic acid. Thus, the emission with peaks at 285 and 315 nm can be assigned to the S2 → S0 transition, and that at 440 nm can be attributed to the S2 → T1 /S1 transition. Although the emission with peaks at 315 and 440 nm originates from the same state S2 of propiolic acid, the measured lifetimes of the emitting states are different (Table 1). This implies that the emission at 315 and 440 nm, respectively, originates from the vibrationally excited and relaxed states of S2 . The decay of fluorescence at the 490-nm peak is a double exponential, implying contribution from different species. The slow component, with a lifetime of 1400 ± 100 ns, can probably be assigned to the T1 → S0 transition of propiolic acid. The fast component, with a lifetime of 120 ± 20 ns, is ascribed to the Swan band transition of C2 . The measured radiative lifetime for the upper state of the Swan band transition (d3 g state) agrees with the literature values of 92–106 ns [34] and 120 ± 10 ns [35]. Even the quenching rate coefficient (1.6 ± 0.1) × 10−10 cm3 molecule−1 s−1 of fluorescence by propiolic acid is in good agreement with that of C2 (d3 g state) by acetylene (1.2 × 10−10 cm3 molecule−1 s−1 ) and ethylene (1.5 × 10−10 cm3 molecule−1 s−1 ) [34]. The excited state of C2 can be produced as a secondary product from the photoproduct C≡CH [36,37], which is generated either as a primary product from the C–C bond cleavage of PA, or as a secondary product from HC≡CCO (primary product of the C–O bond cleavage of PA) after it decays to give CO as a co-product. To understand the feasibility of C≡CH formation after CO elimination from HC≡CCO, we calculated the TS structure for this reaction, which gives an activation barrier of 42.5 kcal mol−1 , in good agreement with the prediction of 45.8 kcal mol−1 [37]. Thus, in our experiment, with the total available energy of only 42.7 kcal mol−1 , the decay of HC≡CCO to produce C≡CH and CO is not possible. Hence, C2 (d3 g state) is generated in the photolysis of PA at 193 nm from the primary product C≡CH.

molecules (monomer and dimer of the acid). As the pressures used were very low in propiolic acid, the contribution from the dimer is expected to be negligible. The plot of ln(I0 /I) versus the propiolic acid pressure was found to be linear, indicating negligible contribution from the dimer. From the slope of the above plot, the absorption cross-section was obtained for propiolic acid at 193 nm to be (1.1 ± 0.2) × 10−17 cm2 molecule−1 . In the case of acetic acid, the contribution from the dimer in the absorption is substantial, here according to the Beer’s law I = exp[−(σm nm + σd nd )l] I0

(12)

where σ m , nm and σ d , nd are the cross-sections and number densities of the monomer and dimer, respectively and l is the absorption path length. The number densities of monomer and dimer were calculated from the known dimerisation equilibrium constant [38]. Absorption cross-sections for monomer and dimer of acetic acid are found to be (1.1±0.2)×10−19 and (2.0±0.4)×10−19 cm2 molecule−1 , respectively. 3.10. Quantum yield of OH formation The concentration of OH produced in molecules per cm3 on 193 nm photolysis of acid is given by 193 193 [OH]photo = σabs (acid) × [acid] × Fphoton × 193 OH (acid)

(13) 193 is the absorption cross-section of acid at 193 nm where σabs 2 in cm molecule−1 , [acid] the concentration of acid in 193 molecules cm−3 , Fphoton the photon fluence in photons

cm−2 and 193 OH is the quantum yield for OH formation. The OH formed in the photolysis was thermalised using high argon pressure (2–5 Torr), and probed at a pump-probe delay OH ) at the time of 90 ␮s. The OH fluorescence signal (Sphoto chosen transition is proportional to the OH concentration OH = C × [OH]. Thus, Eq. (13) can be by the factor Sphoto modified as follows: OH = C × [OH]photo Sphoto 193 193 = C × σabs × [acid] × Fphoton × 193 OH (acid).

3.9. Absorption cross-section The absorption cross-sections for acetic and propiolic acid at 193 nm were measured using a 50-cm long absorption cross-section cell. For this purpose the ratios of intensities, I/I0 , were measured, where I is the laser intensity with the sample and I0 without the sample in the cell. In the case of propiolic acid, the absorption was very strong, and measurements were performed at low pressures, e.g. 10 mTorr onwards. But appreciable absorption for acetic acid was obtained only at pressures greater than 200 mTorr. I/I0 ratios were measured at different propiolic and acetic acid pressures. The absorption contains contributions from both the

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(14)

We have used the reactions: N2 O + hν → O(1 D) + N2 and O(1 D) + H2 O → 2OH to produce a well-defined [OH], as the absorption crosssection of N2 O (9 × 10−20 cm2 ) is well known, and it dissociates to O(1 D) with unit quantum yield [39]. The O(1 D) + H2 O reaction proceeds at a diffusion controlled rate, and thus all of the O(1 D) formed on N2 O photolysis is converted to

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OH by reaction with H2 O for [H2 O] > 10 × N2 O. This reference reaction was also carried out at argon pressure, with a pump-probe delay time, as used in the acid photolysis experiments. Under the above conditions, the OH fluorescence OH is given by signal at a chosen transition, Sref OH = C × [OH]ref Sref 193 193 = C × σabs (N2 O) × [N2 O] × Fphoton × 193 OH (ref).

(15) Maintaining the laser fluence constant, dividing Eq. (14) by Eq. (15) and rearranging, the expression for 193 OH (acid) becomes 193 OH (acid) =

OH × σ 193 (N O) × [N O] × 193 (ref) Sphoto 2 2 OH abs 193 OH σabs (acid) × [acid] × Sref

(16) Thus, the quantum yield for the formation of OH at 193 nm photolysis of acetic acid was found to be 0.8 ± 0.2.

wavelengths studied are identical. In the 218-nm photolysis of acetic acid, Guest and co-workers [17], on the basis of an energy constraint, have established reaction (1) to be the dominant channel for the OH production. This energy constraint was derived from the observed OH fragment translational energy and the known thermochemistry of acetic acid and its products. Owrutsky and Baronavski [11] have found a lifetime of 5 ± 1 ps for the acetyl fragment formed from 194.5-nm photolysis of acetic acid. Assumption of a similar energy distribution as that at 200 and 218 nm yields an average internal energy of 19.9 kcal mol−1 in the acetyl fragment. The estimated lifetime of 4.1 ps at the above internal energy matches quite well with the observed lifetime. This implies that the dominant channel for the OH production at 194.5, 200 and 218 nm is the same, and OH has a sibling acetyl radical. Our work at the excitation energy close to that used by Owrutsky and Baronavski [11] supports that the internal energy in the acetyl radical used by these workers for estimating lifetime is appropriate. All of the above work on the photodissociation of acetic acid can be correlated only on the assumption that the dominant channel for OH production is reaction (1).

4. Discussion 4.1. Photodissociation dynamics of acetic acid 4.1.1. Dominant photochemical path to OH Two possible channels (reactions (1) and (2)) lead to the product OH on photolysis of acetic acid. In Table 2, the energy distributions in photofragments at 200, 218 and 193.3 nm photodissociation of acetic acid are summarised, along with the energy distribution estimated by Owrutsky and Baronavski [11] at 194.5 nm. The fractions of the available energy in OH rotation in photolysis at 218, 200 and 193.3 nm are 0.057, 0.043 and 0.043, respectively. Similar energy distributions in the product OH on the photolysis of acetic acid at 218, 200 and 193 nm indicate that the reaction pathways for OH generation at all three excitation Table 2 Energy distribution in acetic acid photodissociationa,b Wavelength (nm) Eavl  Etrans expt Etrans vibret,hybrid Erot ,OH expt Erot ,OH vibret,hybrid

218.0c 20.8

200.0d 32.7

194.5e 36.7

13.7 ± 1.4 13.2

14.5 ± 1.4 14.0

14.8 14.2

1.2 ± 0.1 0.9

1.4 ± 0.1 1.4

1.6 1.6

193.3f 37.6 +5.0 10.0−2.8 14.3

1.6 ± 0.2 1.6

For vibret, hybrid: the exit barrier energy is partitioned using the impulsive model [40,45], and 80% of excess energy above the barrier is retained in the C=O moiety and 20% is partitioned statistically. a All energies are in kcal mol−1 , assuming a dissociation energy of 110.3 kcal mol−1 . b A 13 kcal mol−1 barrier to dissociation for acetic acid is assumed. c From [17]. d From [13]. e Estimated values [11]. f Present work.

4.1.2. Barrier in the dissociation Based on the high translational energy, Guest and co-workers [13,17] suggested the existence of a barrier in the photodissociation process leading to the OH product. The barrier height is equivalent to the activation energy required for the combination of the products, CH3 CO + OH, but, conversely, it is approximately the same as translational energy released in the dissociation. The photolysis energy should have no effect on the barrier, and thus, the fragment translational energy will change only slightly with the photolysis energy. The good agreement, within error limits, of the observed translational energies for the photofragments formed during 218, 200 and 193 nm photolyses of acid, assuming a barrier for the reaction, supports the existence of the barrier. For reaction (1), which is a simple bond-fission reaction, the existence of the barrier is unexpected. The barrier for such a reaction can arise either from the intermediate intramolecular or from symmetry considerations of the reactant and products. Guest and co-workers [13] have considered the intramolecular keto-enol tautomerisation, in which a hydrogen atom is transferred intramolecularly from the methyl group to the carbonyl oxygen as a possible source for the energy barrier. But however, these workers ruled out this possibility on the basis of the results obtained in the photolysis of CD3 COOH, where they observed only OH product instead of both OH and OD. Thus, it appears that the barrier in the photolysis of acetic acid to produce OH is due to symmetry constraints. 4.1.3. Models of the dissociation dynamics All of the models proposed to explain the observed dynamic photodissociation information range between the statistical model at one end and the impulsive at the other end.

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In the statistical model, the available energy is distributed among all of the accessible states with equal probabilities, while in the impulsive model the distribution of energy among product states is governed by the dissociative event, i.e., by the repulsive forces acting during the breaking of the parent molecule into products. In a photodissociation reaction without a barrier, the partitioning of the available energy can be described, in general, using the statistical model. However, in the case of a reaction with a barrier, North et al. [40] employed a barrier impulsive model. In this model, the total available energy (Eavl ) is distributed by the release of the exit barrier energy (Eimp ). In the statistical model, Eavl is distributed by the release of the excess energy above the barrier (Estat ). The total energy partitioned in different states is obtained by adding the contribution to the state from the statistical and impulsive models. Using the above approach, North et al. [40] have compared the estimated energy partitioned into the product states with experimental results in the photodissociation of acetic acid at 200 and 218 nm [13,17]. Although favourable agreement of the model to the experimental results suggest applicability of the barrier impulsive model for systems having an exit barrier, the consistent overestimation of internal energy of the OH product by the model indicates more complexity in energy partitioning in the acetic acid photolysis. From the theoretical work of Ng and Bell [41] on formic acid, one expects that the 1 (n, ␲∗ ) transition should promote the electron localised on the non-bonding carbonyl oxygen to the antibonding carbonyl carbon, thus, giving pyramidal geometry in the excited state, as compared to the planar ground state geometry. In the pyramidal geometry, the carbonyl C=O bond is extended, if we assume that the 1 (n, ␲∗ ) transition in acetic acid is similar to that of formic acid, and we expect elongation of C=O bond by localised excitation, as in the case of formic acid. In this case, the Frank–Condon principle would encourage the C=O to be locally vibrationally excited in the excited acetic acid. If the subsequent dissociation of the excited acetic acid is adiabatic, then the C=O moiety in the product would also be in the vibrationally excited state. Several N=O containing molecules when excited in their 1 (n, ␲∗ ) states, dissociate to give translationally hot products along with retention of the vibrational energy deposited in the N=O mode of the excited parent molecules as vibrational energy of the free NO fragment [27,42–44]. Such retention is attributed to the overlap of the vibrational factor between the initial NO oscillator in the excited parent molecule and that of the free fragment [44]. We have assumed a similar retention in the C=O moiety to the extent of 80% of the excess energy above the barrier and performed the energy partitioning using a model where the exit barrier energy is partitioned impulsively [40,45] and the remaining energy (20% of the excess energy above the barrier) is distributed statistically. The results, presented in Table 2, are in good agreement with the experimental values of energy partitioned in OH rotation and product translation. Thus, it appears to be quite probable that vibrational excitation is localised in the C=O moiety. Fur-

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ther experiments are definitely needed to probe the localisation of vibrational excitation in the C=O moiety of either the primary CH3 CO fragment or its subsequent CO fragment. 4.2. Photodissociation dynamics of acrylic acid 4.2.1. Nature of the dissociative state leading to OH formation Acrylic acid has been studied in detail to obtain the structures of the ground state and a few of its excited states [12,24,25]. Acrylic acid in its ground state has four isomers, with the s-cis conformer having the lowest energy. The structure of the most stable conformer is given later. The structural parameters for the ground state and excited states are described in references [12,24,25].

The lowest triplet state, T1 , is accessible from the ground state through a 3 (␲∗ → ␲) transition, whereas the next higher triplet state, T2 , is accessible via a 3 (␲∗ → n) transition. In the former, an electron initially excited from the ␲ to the ␲∗ orbital elongates the double bond (C1 =C2 ), making the structure of the T1 minimum a distorted type. The T1 state correlates fully with reaction (3) of acrylic acid dissociation. The structural feature of the transition state at the T1 surface also predicts that it is responsible for the reaction (3) giving HOCO and CH2 CH. In the case of the T2 transition, an electron is excited from the n-orbital to the C3 =O1 ␲∗ -orbital. Such a localised transition weakens the C3 =O1 bond and increases the contribution of single-bond character. To stabilise the system, a ␲-bond is formed between C2 and C3 . The overall changes in the T2 state makes the C1 –C2 =C3 –O1 backbone rigid and make its cleavage very difficult. The only possibility left is the cleavage of the C3 –O2 bond to give OH as one of the products (reaction (4)). In fact, this state does correlate with the products CH2 CHCO and OH. The transition state calculated on this surface clearly indicates the formation of OH. Acrylic acid in the S1 state (a 1 (n → ␲∗ ) transition) has a structure similar to the T2 state. Since the T2 and S1 states have similar structures, they behave similarly with respect to dissociation. Absorption of a 193-nm photon by acrylic acid excites the molecule to the S2 state. This 1 (␲ → ␲∗ ) transition is centred at vinyl group and has a mixed character [␲∗ (C=O) and ␲∗ (C=C)]. According to the structure calculated by Kitchen et al. [14], a large amount of electron density is transferred from the double bond to the COOH group, which can be treated as intramolecular charge transfer. This structure is so different from the ground state that the double bond is

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actually shifted to the next carbon atom and the initial double bond is elongated and behaves as a single bond. Such a drastic change in the geometry of the molecule closes off the direct dissociation mechanism from the S2 state, which would lead to the C–C bond rupture. The S2 state correlates with the electronically excited products, but energetically it is not feasible for 193-nm photolysis. Hence, this state decays through different paths, via radiationless or radiative transitions. The radiationless routes, namely, internal conversion (IC) to S0 and S1 and intersystem crossing (ISC) to T2 , are important. The states S1 and T2 correlate with the OH-forming channel (4). Hence, we will consider only these states for further discussion. If the S1 state is populated from S2 through IC, it will undergo a direct dissociation, giving CH2 CHCO(2 A ) and OH(2 ) (reaction (4)). Another mechanism for OH formation is via ISC from the S2 to the T2 state, followed by fast dissociation (reaction (4)). There is an alternative indirect process to populate the T2 state from the initially prepared S2 state via S1 , which is not feasible, as the S1 /T2 ISC cannot compete with the direct dissociation on the S1 surface [46]. Rotational state distributions of OH formed after 193 and 248 nm photodissociation show marked differences. The higher value of the spin–orbit ratio (F1 /F2 ) obtained at both wavelengths suggests a coupling between the dissociative states with a nearby triplet state. Such behaviour is well known in the photodissociation of molecules like NCNO [47] and NO2 [48]. For 193-nm photolysis, the 3/2 state is populated more predominately as compared to that at 248 nm. Excitation at 248 nm leads to the dissociative state S1 , and the preferential population of 3/2 is due to strong coupling of the S1 state with the T1 state [12]. For 193-nm photolysis, the observed higher spin–orbit ratio may be due to strong mixing of higher vibrational levels of the S1 state (formed from the S2 state by IC) with the T2 state or dissociation from the T2 state formed directly from the S2 state through ISC. The relative population of the Λ-doublet of the OH fragment provides the exit channel dynamics in the bond cleavage process. As the ratio, (A )/(A

) is the same (0.8) at both the wavelengths studied, exit channel dynamics of the OH reaction channel seem to be similar. The non-statistical ratio of the Λ-doublet suggests that the transition state for reaction (4) (OH-forming reaction) is slightly non-planar in nature. The optimised structures of the transition states, obtained by an ab initio method on both S1 and T2 surfaces are non-planar, which further supports the proposed dissociation mechanism. Since the structures of the transition states on the T2 and S1 surfaces are very similar, it is not surprising that the Λ-doublet ratio for both dissociation wavelengths is the same. 4.2.2. Existence of an exit channel barrier in the OH-forming reaction The translational energy released into the product states for 193 and 248 nm photolysis are 13.2 ± 3.1 and

10.2 ± 2.8 kcal mol−1 , respectively. The available energy for partitioning among product states formed on photolysis at 248 and 193 nm are 19 and 52 kcal mol−1 , respectively, assuming ∼96 kcal mol−1 is the enthalpy change for the dissociation channel (reaction (4)). However, the translational energy released on photolysis at 193 nm is 13.2 ± 1.8 kcal mol−1 , only 3 kcal mol−1 higher than that on photolysis at 248 nm. Such closeness in the energy released in the translational mode for 193 and 248 nm photodissociation indicates the presence of an exit channel barrier at both excitation energies. For a reaction with an exit channel barrier, North et al. [40] have proposed a barrier impulsive model for the distribution of available energy. In this model, the energy equivalent to the barrier height is partitioned by the impulsiveness of products. The impulsive model [45] predicts that almost 95% of the exit channel barrier energy will be released in the product translational mode in reaction (4). The energy above the barrier is distributed statistically among the different modes of the photofragments. The relative translational energy in products for 248-nm photodissociation is 10.2 kcal mol−1 , which predicts an exit channel barrier height of about ∼11 kcal mol−1 . At 248 nm, the dissociation takes place solely on the S1 state, and thus, the above barrier of 11 kcal mol−1 lies on the S1 state at the exit channel. Our estimated values for the exit channel barrier on the S1 surface are in good agreement with the value calculated by Fang [24] and Fang and Liu [25]. Ab initio calculations predict the exit channel barrier height on the T2 surface to be similar to that on the S1 surface. 4.2.3. Assignment of fluorescence for 193-nm excitation The decay rate constants for all of the fluorescence peaks are equal, within experimental error. This implies that all of the observed peaks arise from the same emitter. The emission is attributed to the S2 state on the basis of energy involved in the transition. The three observed peaks are assigned to different transitions from the S2 state as described in the following sections. 4.2.3.1. Fluorescence (275–305 and 290–345 nm range) with λmax at 290 and 318 nm. The fluorescence in this range is assigned to the S2 → S0 transition on comparing the energy of the emitted photon with the energy levels of states reported by Fang [24]. The S2 state geometry is quite different from the S0 state, and leads to a vertical transition from the S2 state to a higher vibrational state of S0 , red-shifted, from the (0, 0) transition. 4.2.3.2. Fluorescence (380–500 nm range) with λmax at 438 nm. The emission in the 380–500 nm range can be assigned to either the S2 → S1 or S2 → T1 transitions. The observed fluorescence at lower wavelength in this range is difficult to assign to S2 → S1 due to energy mismatch. The S2 state has a mixed character [␲∗ (C=O) and ␲∗ (C=C)] and can thus enhance the spin–orbit coupling, leading to an increase in the transition oscillator strength of the spin-forbidden S2 → T1

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transition. Further work is needed to unambiguously assign the observed fluorescence in this range. 4.3. Photodissociation dynamics of propiolic acid 4.3.1. Energy partitioning The major fraction of the available energy is channelled into the translational energy of the photofragments, which can result from either the impulsive dissociation of the HCCCO–OH bond of propiolic acid, or the presence of an exit barrier. Because an impulsive dissociation occurs from a repulsive PES with a high dissociation rate coefficient, in the present case, with a slow formation of OH, the impulsive dissociation is not responsible for a high fraction of the available energy being partitioned into the translational energy of the products. Thus, there should be an exit barrier to the C–O bond cleavage. Such an exit barrier is observed for a simple bond cleavage leading to OH in the photodissociation of acetic [8,13] and acrylic acids [14,15]. The results of partitioning of the available energy among various degrees of freedom of the two photofragments can be examined qualitatively in terms of two limiting models, the impulsive [45] and the statistical [49]. Although the impulsive model favours usually high translational fragment energy, it cannot explain the observed slow formation of OH from the S2 state in the present work. The observed Boltzmann distributions of the rotational and vibrational state distributions of OH(v, J) suggest the statistical nature of the fragment state distribution. However, the statistical model assumes a complete randomisation of Eavl among all degrees of freedom prior to dissociation. In the present work, a complete randomisation of energy has not taken place, since the vibration, rotation and translation of OH have different temperatures. This implies that a hybrid model [40,50], consisting of both the impulsive and the statistical models should better explain the observed state distribution. To validate our qualitative prediction based on experimental results, we employed both statistical and impulsive models to calculate partitioning of the available energy of 42.7 kcal mol−1 into various degrees of freedom of photofragments OH and HC≡CCO. According to the results shown in Table 3, the statistical model grossly underestimates the available energy appearing as the relative kinetic energy of the products, and overestimates the internal energy Table 3 The partitioning of the available energya into translation and internal modes of the photofragments (OH and HC≡CCO) of propiolic acid at 193 nm Etrans (OH + HC≡CCO) Er (OH) Ev (OH) Eint (HC≡CCO) a b

Observed

Statistical

Impulsive

Hybridb

32.2 1.5 0.1 8.9

6.6 2.3 1.0 32.8

22.8 1.0 0.1 18.8

32.2 1.8 0.0 8.7

All energies are in kcal mol−1 . Eavl = 42.7 kcal mol−1 . Assuming an exit barrier of 37.5 kcal mol−1 .

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of HC≡CCO, and thus fails to explain the observed partitioning of the available energy among the photofragments. The other limiting model, the impulsive model, is based on the assumption that the amount of available energy acts as a repulsive potential along the breaking bond. The conservation of linear and angular momenta determines the energy partitioning among fragments. Like the statistical model, the impulsive model is also unable to explain well the measured data on energy partitioning between fragments. However, the prediction based on the impulsive model is relatively closer to the experimental values (Table 3). Both these models predict low rotational and vibrational energy going into OH, as observed. The failure of both the statistical and impulsive models in explaining the partitioning of the available energy prompted us to apply the hybrid model [40,50]. The energy measured into fragments is reproduced well with an assumed exit barrier of 37.5 kcal mol−1 . This model predicts almost no vibrational excitation of OH. The origin of a barrier in the exit channel of the C–O bond cleavage of a carboxylic acid can be due to dissociation from its enol form or to some features, such as non-adiabatic surface-crossing, of the excited state potential energy surfaces [12]. In the absence of an ␣-H atom on the keto group of propiolic acid, its enol form does not exist. Thus, the surface crossing of the excited PES to which propiolic acid is excited initially should be mainly responsible for a high exit barrier (37.5 kcal mol−1 ). In order to fully understand the photodissociation dynamics of propiolic acid, more experiments and higher levels of ab initio calculations of potential energy surfaces are necessary. 4.3.2. Dissociative state of propiolic acid The excitation of propiolic acid at 193 nm populates the S2 excited electronic state through the 1 (␲, ␲∗ ) transition, similar to acrylic acid [24,25]. The excited molecule can relax through radiative decay, non-radiative decay or can undergo photodissociation. The relative importance of these relaxation processes depends on their relative rate coefficients. The slow formation of OH suggests that other radiative and non-radiative processes can compete with dissociation. Thus, fluorescence (250–600 nm) from electronically excited propiolic acid can be observed. Further, the slow dissociation rate implies that the dissociation occurs from a bound PES and not from a repulsive one, if accessed directly. The high absorption cross-section of propiolic acid at 193 nm implies a short radiative lifetime of the S2 state. Thus, the slow dissociation of propiolic acid precludes the initially excited S2 state to be the dissociative state. Is this dissociative state the ground electronic state? It is suggested that, after excitation of propynal to high-lying electronic states at 193 nm, IC or ISC precedes the dissociation process. Dissociation of propiolic acid after excitation at 193 nm is expected to be similar to propynal [37]. However, the C–O bond fission imparts a significant amount of energy into relative fragment translation, indicating the presence of an exit barrier.

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This suggests that dissociation occurs on an electronically excited potential energy surface, since a simple bond cleavage in the ground electronic state is generally barrierless. Information on whether a molecule dissociates from a triplet potential energy surface can be obtained from the ratio of the spin–orbit states. A triplet dissociative state can change the spin–orbit ratio in favour of the 2 3/2 state. Although there is some preference for the 2 3/2 state of OH produced after photolysis of PA at 193 nm, this result alone is not sufficient to conclude unambiguously that PA dissociates from a triplet state. The slow formation of OH can also result from the secondary dissociation of CO–OH, which is formed after the HCC–COOH bond cleavage. However, the energetics do not permit the secondary OH carrying away the high translational energy (26 kcal mol−1 ). After the C–C bond cleavage (with the dissociation energy of ∼116 kcal mol−1 )1 of propiolic acid, the available energy with the HCC and HOCO fragments is about 32 kcal mol−1 . With the dissociation energy of the HO–CO bond of 36.0 kcal mol−1 (see footnote 1), HOCO will remain stable and cannot channel the observed 21 kcal mol−1 of translational energy into OH.

5. Conclusions The dynamics of OH formation have been studied in photoexcited acetic, acrylic and propiolic acids. The partitioning of energy was mapped by probing the intensities and Doppler profiles of ro-vibronic lines. The photofragment OH is formed mainly in the ground vibrational state, except for propiolic acid photodissociation, where we were able to monitor the OH formed in the v = 1 state. The major part of the available energy was partitioned into the relative translation of the photoproducts, indicating the presence of an exit barrier on the dissociative surface. Although decarboxylation is the major channel from the ground electronic state of carboxylic acids, in the present studies of the electronically excited carboxylic acids, OH formation is the predominant channel. This result, in combination with the high translational energy in the photoproduct, indicates that the ISC rate from the initially prepared excited state to the ground state is very slow, and the excited molecule dissociates either from the initially prepared or another excited state. As we go from the saturated acid (acetic) to the highly unsaturated acid (propiolic), the amount of energy partitioned into the internal states of the OH photofragment increases, implying that an unsaturated bond facilitates the randomisation of the available energy. In the case of acetic acid, some part of the 1 The heats of formation (in kcal mol−1 ) used to estimate the enthalpy of a reaction are H ◦f (HC≡C–COOH) = −35.6 (estimated using the heats of formation of acrylic acid, propenal and propynal); H ◦f (HC≡CCO) = 61.1, H ◦f (HC≡C) = 134.3; H ◦f (HC≡CCHO) = 25.8 and H ◦f (H2 C=CHCHO) = −15.2 (from [28]); H ◦f (H2 C= CHCOOH) = −76.6 (from [10]); H ◦f (CO) = −26.4, H ◦f (OH) = 9.4 and H ◦f (HOCO) = −53.3 (from [51]).

available energy is localised in the initially excited bond. In acetic acid photodissociation, the OH is formed during the laser pulse, whereas, in the case of acrylic acid and propiolic acid for 193-nm dissociation, the OH is formed with risetimes of 2.0 and 0.5 ␮s, respectively, with preference for the 2 3/2 spin-obit state. These results indicate that the unsaturated C–C bond promotes the mixing of the excited singlet state with the nearby triplet state, thus slowing down the dissociative process. This is further substantiated by the strong visible fluorescence observed on photoexcitation of acrylic and propiolic acids at 193 nm, which is absent for acetic acid. References [1] G. Wlodarczak, D. Boucher, J. Burie, J. Demaison, The millimetrewave spectrum of propiolic acid, J. Mol. Spectrosc. 123 (1987) 496– 498. [2] D.G. Lister, J.K. Tyler, The conformation and dipole moment of propiolic acid deduced from its microwave spectrum, Spectrochim. Acta A 28 (1972) 1423–1427. [3] E.E. Barnes, W.T. Simpson, Correlations among electronic transitions for carbonyl and carboxyl in the vacuum ultraviolet, J. Chem. Phys. 39 (1963) 670–675. [4] D.L. Singleton, G. Parasevopoulos, R.S. Irwin, UV absorption crosssections of the monomer and dimer of formic acid, J. Photochem. 37 (1987) 209–216. [5] P.E. Hintze, S. Aloisio, V. Vaida, Electronic spectroscopy of organic acid dimmers, Chem. Phys. Lett. 343 (2001) 159–165. [6] K. Saito, T. Sasaki, I. Yoshinobu, A. Imamura, Thermal decomposition of ethyl acetate. Branching ratio of the competing paths in the pyrolysis of the produced acetic acid, Chem. Phys. Lett. 170 (1990) 385–388. [7] M.T. Nguen, P. Ruelle, Comments on ab initio quantum-chemical study of the unimolecular pyrolysis mechanism of acetic acid, Chem. Phys. Lett. 138 (1987) 486–488. [8] P.D. Naik, H.P. Upadhyaya, A. Kumar, A.V. Sapre, J.P. Mittal, Dynamics of acetic acid dissociation at 193.3 nm: selectivity in OH reaction channel, Chem. Phys. Lett. 340 (2001) 116–122. [9] H.T. Kwon, S.K. Shin, S.K. Kim, H.L. Kim, C.R. Park, Laser photofragment spectroscopy of the NO2 dissociation at 337 nm: a nonstatistical decay process, J. Phys. Chem. A 105 (2001) 6775–6779. [10] H. Su, Y. He, F. Kong, W. Fang, R. Liu, Photodissociation of formic acid, J. Chem. Phys. 113 (2000) 1891–1897. [11] J.C. Owrutsky, A.P. Baronavski, Ultrafast photodissociation studies of acetyl cyanide and acetic acid and unimolecular decomposition rates of the acetyl radical products, J. Chem. Phys. 111 (1999) 7329– 7336. [12] M.F. Arendt, P.W. Browning, L.J. Butler, Emission spectroscopy of the predissociative excited state dynamics of acrolein, acrylic acid, and acryloyl chloride at 199 nm, J. Chem. Phys. 103 (1995) 5877–5885. [13] S.S. Hunnicutt, L.D. Waits, J.A. Guest, 1 (n, ␲∗ ) Photochemistry of acetic acid at 200 nm: further evidence for an exit channel barrier and reaction, J. Phys. Chem. 95 (1991) 562–570. [14] D.C. Kitchen, N.R. Forde, L.J. Butler, Photodissociation of acrylic acid at 193 nm, J. Phys. Chem. A 101 (1997) 6603–6610. [15] H.P. Upadhyaya, A. Kumar, P.D. Naik, A.V. Sapre, J.P. Mittal, The OH formation dynamics in the dissociation of acrylic acid in its (n, ␲∗ ) and (␲, ␲∗ ) transitions excited at 248 and 193 nm, J. Chem. Phys. (in press). [16] A. Kumar, H.P. Upadhyaya, P.D. Naik, J.P. Mittal, Photodissociation dynamics of propiolic acid at 193 nm: the state distribution of the nascent OH product, J. Phys. Chem. A (in press).

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Hari P. Upadhyaya was born in 1965 in Guwahati, India. He received his BSc in chemistry in 1987 from North Eastern Hill University, Shillong, India and MSc in chemistry in 1990 from University of Delhi, India. He is currently working as scientific officer in RC & CD Division of Bhabha Atomic Research Centre, Mumbai, India. He received his PhD degree in chemistry from University of Mumbai in 2001. He worked for a period of 1 year at Institute of Physical Chemistry, University of Heidelberg, Germany in the area of photodissociation and reaction dynamics with Prof. J. Wolfrum during 1997–1998. His current research interests focus on photodissociation and reaction dynamics involving small polyatomic molecules as well kinetics of atmospherically important reactions in gas phase.

Prakash D. Naik was born in 1959 in Karwar, India. He received his BSc in chemistry in 1980 from Karnataka University, Dharwar, India and MSc in chemistry in 1983 from University of Mumbai, India. He is currently working as scientific officer in RC & CD Division of Bhabha Atomic Research Centre, Mumbai, India. He received his PhD degree in chemistry from University of Mumbai in 1992. He worked for a period of 1.5 years at Institute of Physical Chemistry, University of Heidelberg, Germany in the area of photodissociation and reaction dynamics with Prof. J. Wolfrum during 1992–1993. His current research interests focus on laser-induced dissociation dynamics involving small polyatomic molecules and kinetics of atmospherically important reactions in gas phase.

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Awadhesh Kumar was born in 1964 in Bihar, India. He received his BSc in chemistry in 1985 from University of Delhi, India and MSc in chemistry in 1987 from Indian Institute of Technology, Kanpur, India. He is currently working as scientific officer in RC & CD Division of Bhabha Atomic Research Centre, Mumbai, India. He received his PhD degree in chemistry from University of Mumbai in 1995. He worked for a period of 2 years at National Tsing Hua University, Hsinchu, Taiwan with Prof. Yuan Pern Lee. His current research interests focus on dynamics of gas phase reactions induced by lasers using resonant four-wave mixing and laser-induced fluorescence with special reference to atmospherically important free radical species.

Avinash V. Sapre was born in 1943 in Karad, Maharashtra State, India. After receiving BSc (1962) and MSc (1964) degrees from University of Pune in chemistry he joined Bhabha Atomic Reseacrh Centre, Mumbai. He received PhD from Mumbai University in 1982. He is currently working as scientific officer in RC & CD Division of Bhabha Atomic Research Centre, Mumbai, India. He worked for a period of 1 year at Max Planck Institute fuer Stroemungs-

forschung, Goettingen, Germany during 1984–1985. His current research interests focus on laser-induced reaction dynamics in gas and liquid phases, photoprocesses of fullerenes and dissociative electron transfer reactions in solutions.

Jai P. Mittal was born in 1940 in Meerut, India. After receiving his MSc degree in chemistry from Meerut University, India in 1959, he joined Bhabha Atomic Research Centre, Mumbai, India. He received his PhD degree in chemistry from University of Notre Dame, USA in 1967 and was a post-doctor fellow in the University of California, Los Angeles with Nobel laureate Prof. W.F. Libby during 1968–1969. He was a visiting scientist at US Army Laboratory, Natick (1971–1972) and University of Notre Dame (1981–1982). He is fellow of Indian Academy of Sciences, India, National Academy of Sciences, India and Indian National Science Academy and Third World Academy of Sciences Trieste, Italy. His current interests are radiation chemistry of cation radicals, laser-induced processes in liquids and gases, ultrafast phenomena, photo- and radiation-chemistry of fullerenes and biologically important molecules. At present he is the Director of Chemistry & Isotope Group, BARC.