Roaming Dynamics in the Photodissociation of Formic Acid at 230 nm

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Roaming Dynamics in the Photodissociation of Formic Acid at 230 nm Yujie Ma, Jiaxing Liu, Fangfang Li, Fengyan Wang, and Theofanis N. Kitsopoulos J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b00724 • Publication Date (Web): 10 Apr 2019 Downloaded from http://pubs.acs.org on April 10, 2019

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The Journal of Physical Chemistry

Roaming Dynamics in the Photodissociation of Formic Acid at 230 nm Yujie Ma, a Jiaxing Liu,a Fangfang Li,a Fengyan Wang a*, and Theofanis N. Kitsopoulos b* a

Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysts and Innovative

Materials, Collaborative Innovation Centre of Chemistry for Energy Materials (iChEM), Fudan University, Shanghai, 200433, P. R. China. b

Department of Dynamics at Surfaces, Max Planck Institute for Biophysical Chemistry,

Göttingen, Germany; Institute of Electronic Structure and Laser, FORTH, Heraklion, Greece; Department of Chemistry, University of Crete, Heraklion, Greece Corresponding Authors *[email protected]; [email protected]

Keywords: Reaction dynamics, roaming, photodissociation, slice imaging, formic acid

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Abstract Roaming dynamics is observed in the photodissociation of formic acid (HCOOH) at 230 nm by using the slice imaging method. In combination with rotational state selective (2+1) resonance-enhanced multiphoton ionization of the CO fragments, the speed distributions of the CO fragments exhibit a low recoil velocity at low rotational levels of J = 9 and 20, while the velocity distributions of CO at high rotational levels of J = 30 and 48 show a relatively large recoil velocity. The experimental results indicate that the roaming of OH radical should be related with the formation of CO + H2O channel at the present photolysis energy. Unlike the roaming pathways occuring in H2CO that can be described by loose flat potential, our CO speed distribution analysis suggests the presence of a “tight” flat potential in the roaming dynamics of HCOOH molecules.

1. INTRODUCTION In 1993, van Zee et al. using laser-induced fluorescence reported that a significant fraction of the CO(v = 0) photofragment is found in low rotational states (Jco < 15) when formaldehyde (H2CO) is excited above the threshold of the H + HCO dissociation channel.

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In 2004, Townsend et al. using slice imaging revealed that this dissociation

pathway proceeds the “roaming” dynamics of hydrogen atom to yield rotationally cold CO in conjunction with highly vibrationally excited H2.2 They defined the dissociation mechanism as roaming, because of the trajectory behaviour of the intermediate H atom. The H atom separates from the molecule following the radical channel, but instead of dissociating it spends a considerable amount of time near the molecule, eventually abstracting the remaining H atom from the molecule to form H2.

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Undoubtedly, the


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dynamical signatures of products formed by this roaming mechanism are very different from the ones formed via the conventional Transition State (TS). In the case of the conventional saddle point mechanism, the TS is a well-defined energetic bottleneck. However, the TS in the roaming channel occurs on a relatively flat region of the potential (loose TS). 3 Thus the roaming mechanism produces fragments that are rotationally colder and with little translational energy, in the absence of any tight TS that could afterwards release the potential energy into fragment kinetic energy. 5 The discovery of roaming dynamics stimulated extensive studies of a large number of molecules. This roaming mechanism has been postulated to explain bimodal product state distributions seen in the photodissociation of CH3CHO,5-9 NO3,10-12 C2H4OH,13 HCOOCH314 and aliphatic aldehydes (RCHO, R = H, CH3, and large alkyl groups)15, 16, etc. The general roaming mechanism is thought to start with radical products on the ground state surface, and it is associated with delayed arrival of products. In addition, observations of abnormal internal energy distributions are similar to the product distributions produced by a direct abstraction mechanism.2, 6, 13 Various types of roaming mechanisms have also been documented. Grubb et al demonstrate that the energetically accessible conical intersections between the A and X states are in the roaming region of the configuration space.10 The recent investigations on the photo-initiated beam reactions for methyl formate (HCOOCH3) conducted in Taipei have also confirmed that the conical intersection plays a key role in the roaming mechanism. 14 In this paper, we investigate the presence of roaming dynamics in the dissociation of formic acid by mapping out the translational energy of CO product in different rotational levels. Formic acid (HCOOH) is one of the simplest organic acids and amongst the most


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abundant pollutants in the atmosphere. It is also an important intermediate in the oxidation of unsaturated hydrocarbons in combustion. Previous experimental studies of the photodissociation of HCOOH mainly focused on 220 -250 nm region,17-27 involving the S0, S1 and T1 potential energy surfaces (PESs). The absorption of an ultraviolet photon promotes the molecule from the ground electronic state, S0, to the first electronically excited singlet state, S1 through the n → π* transition. Electronically excited HCOOH will undergo dissociation into the radical and neutral product channels, as shown below, 28-33

HCOOH → HCO + OH (ΔH0 ≈ 452 kJ/mol)

(1)

→ H + COOH / HCOO(ΔH0 ≈ 359 / 467 kJ/mol)

(2)

→ H2O + CO (ΔH0 ≈ 35 kJ/mol)

(3)

Reaction (1) becomes accessible at ~252 nm (475 kJ/mol). The excited HCOOH in S1 undergoes an intersystem crossing (ISC) to T1, then dissociates according to reaction (1).33 By increasing the excitation energy above 240 nm (499 kJ/mol), direct OH + HCO dissociation via the S1 surface TS dominates.26, appreciable

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The yields of reaction (2) become

at ~253 nm (472 kJ/mol) through the ISC from S1 to T1 and over (or

tunnelling through) a barrier on the triplet potential-energy surface.20 The reaction channel producing HCOO + H requires more energy (~532 kJ/mol) and is beyond the scope of this article along with CO2 + H2 channel.33 The vibrationally excited CO and H2O were recorded in the CO + H2O channel, originating from the dissociation on the S0 surface after internal conversion from the S1 state.22

2. EXPERIMENTAL METHODS


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The time-sliced ion velocity imaging experiment was performed in our crossedmolecular beam setup.34 A single molecular beam was used in this photodissociation experiment.35 Briefly, formic acid vapour was seeded in neon and expanded into the high vacuum via a heated Even-Lavie valve operating at 150 ℃ to reduce cluster formation. I n the centre of the ion optics, the formic acid molecular beam intersects a linearly polarized laser pulse at around 230 nm, generated by frequency doubling the output of an OPO/OPA laser pumped by a Nd:YAG laser (Continuum). The linewidth of the laser is about 0.1 cm-1. The CO (X1Σ+, v = 0, J) fragments were ionized via (2+1) resonanceenhanced multiphoton ionization (REMPI) through the CO (B1Σ+, v = 0, J) intermediate state.36 The ionized CO fragments were accelerated to the position sensitive detector, which is composed of 2 microchannel plates (MCP, 75 mm diameter, Photek) and 1 Phosphor Screen (P43, Photek). Using a 20-30 ns width pulse on the back MCP enables mass gating of CO+. A LaVision Elite CCD camera captures the light emitted from the P43 and transmits it to the computer with an improved software (Davis 8.2) for image acquisition.

3. RESULTS AND DISCUSSION 3.1 Slice images for CO(J) products Figure 1 shows the (2+1) REMPI spectrum of CO (X1Σ+, v = 0, J) fragments via the intermediate state B1Σ+ in our one-colour experiment, with a lower rotational state of CO detected at longer wavelength. The intensity of CO apparently decreases with increasing J, without correcting for laser energy fluctuation. Careful subtraction of the strong, but


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constant, CO+ background in the ensuing data analysis was necessary for the CO fragments in high rotational levels. Rotational levels of CO are in principle easier to resolve at higher rotational states because of the larger energy spacing between adjacent J’s, ΔE ≈ 2JΔB, where ΔB is the energy difference between the rotational constant of CO(X1Σ+, v = 0) and CO(B1Σ+, v = 0), i.e., approximately 0.03 cm-1. For J’s ≥ 20, we observe the Doppler profile in slice images of the CO(J) distributions relative to the laser propagation direction. As shown in Fig. 2, when scanning the wavelength from 230.082 nm to 230.079 nm, we see a clear Doppler shift of CO(J = 20) velocity distributions. The velocity profile of the recorded product is available from its distance and direction to the centre of the velocity-mapped image, as indicated in Fig. 2(a). In the slice image, the CO(J) signal of the same speed flying to the left requires longer REMPI wavelengths than the CO flying rightward with respect to the centre. At 230.082 nm in Fig. 2 (a), CO(J = 20) flying to the left was recorded together with CO (J = 19) flying to the right. As the probe wavelength decreases, a velocity distribution of CO (J = 20) from left to right was detected, as shown in Fig. 2(a)-(d). At 230.0795 nm in Fig. 2 (e), the right component represents CO(J = 20), and the left component with almost equal intensity is CO(J = 21). At 230.079 nm in Fig. 2 (f), the CO(J = 20) became weaker than CO(J = 21). Only the comparison of the intensity of CO in one image is meaningful. Comparing the relative intensities between images directly is not possible because of the completely different accumulation times for images probed at different wavelengths. From the slice images at 230.082 nm and 230.079 nm, production of the CO+ without Doppler effect being observed near the centre most likely stems from the dissociation of formic acid cations, which we regard as background in the current study.


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Figure 3 displays the sliced velocity mapping of CO recorded at various REMPI wavelengths, and the size reduction ratio is the same as that of Fig. 2, which allows direct comparison of the image sizes. At low rotational levels, resolving the closely spaced adjacent rotational states using our REMPI scheme is marginal. For J = 9, the REMPI energy shift between J+1 or J-1 is only about 0.5 cm-1, resulting in no observable Doppler effect. In Fig. 3(a), the slice image of CO(J = 9) shows a strong signal near the centre, and its radius is smaller than that observed in the CO slice image at high J, for example, J = 30. Figures 3(b) and (c) show the slice images of CO(J = 30), where the velocity profiles are mapped on the left and in the centre, respectively, and both show the large speed distributions compared to the image of CO (J = 9). A Doppler profile of CO(J = 29) was also observed at 230.0465 nm, as shown in the right panel of Fig. 3 (b). The slice images of CO(J = 48) in Fig. 3(d)-(f) shows the evolution of Doppler profile at J = 48 at three wavelengths, specifically on the left side of the axis of symmetry, at 229.963 nm, through the centre, at 229.962 nm, and on the right side of the axis of symmetry, at 229.960 nm. For CO(J = 48), the REMPI energy shift between J and J + 1 (or J-1) is about 2.9 cm-1, and when measuring its Doppler distribution (in a range about ± 0.5 cm-1), the influence of CO in the adjacent J’s is negligible. Although the CO(J = 48) signal is very weak compared to the strong background near the centre of the image, the Doppler signal can be clearly seen in the corresponding slice image. The maximum speed distributions of CO(J = 48) did not increase with respect to that of CO(J = 30), indicating a conventional dissociation pathway for the CO formation in J = 30 and 48. Thus, the Doppler images reveal two distinct mechanisms. The faster component of CO for high rotational levels


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probably arises from dissociation via traditional TS, while the slow component of CO at low rotational levels follows a roaming type dissociation mechanism. Fig. 4 shows the Doppler-scanned images obtained for J = 20, 30 and 48, respectively, where the central background has been subtracted. We see an isotropic angular distribution in the Doppler-scanned slice images of the CO fragments. In addition, in the HCO + OH channel, HCO can absorb one extra photon and dissociate into CO products. But this dissociation is a strong parallel anisotropic process

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and can

therefore be excluded from the formation of isotropic CO. The slice image of CO(J = 30) in Fig. 4 shows a faster component than CO(J = 20), consistent with the observations in Doppler images. The CO(J = 48) weakens and shows a distributions similar to CO(J = 30). 3.2 Speed distributions Figure 5 (a) shows the speed distributions of CO fragments obtained from the background corrected centre slice image at 230.101 nm, J = 9, 230.0805 nm, J = 20, 230.0445 nm, J = 30, and 229.962 nm, J = 48, respectively, which were integrated in a small angular range (± 15o from the linear polarization direction of the laser). In the speed distribution for J = 9, we see an intense peak at a slow speed, ~ 400 m/s, and a less intense peak at a relatively fast speed, ~1300 m/s. We cannot rule out the presence of clusters that depend strongly on the molecular beam source conditions, and will influence to slowest channel near the centre. The CO(J = 20) speed distribution shows a very weak peak at ~400 m/s and maximum at ~1500 m/s. With increasing J, the speed distribution’s maximum shifts to 1700 m/s at J = 30 and 48. The roaming dynamics tend to result in slow speed component at J = 9 and 20.


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Figure 5 (b) shows the speed distributions of CO fragments obtained from the Doppler slice image at 230.0795 nm, J = 20 and 21, 230.0465 nm, J = 29 & 30, and 229.960 and 229.963 nm, J = 48, respectively. At each wavelength, we detect only the fast components of CO products. We integrated the CO in the 90o angular angle to obtain the Doppler-shift distributions. The peak at ~ 1150 m/s seen for J = 48 and 30 fitted by the black dashed curve is the background from non-resonant CO+ ions, which can be easily resolved by the resonance detection of CO. The background determination is particularly important when the CO resonance signal becomes weak at high J. The CO Doppler shift speed distributions reveal significant changes in fast component for different J’s. Clearly, most probable velocity at 1500 m/s for J = 20 is less than the most probable velocity of 2200 m/s for J = 30. Based on the conservation of recoil momentum and the conservation of total energy between CO and H2O products, less release kinetic energy in the coproducts observed in CO at small rotational levels is accompanied by a large amount of internal energy in H2O, indicating the roaming dynamics in the formation of CO(J ≤ 20) + H2O channel. Figure 5 (c) shows the speed distributions of CO fragments at J = 20, 30 and 48 obtained with the Doppler-Scanning, which is similar to Fig. 5 (a). It is not possible to subtract the background that accumulates during the Doppler scanning completely. The conversion of the speed distributions into the total translational energy distributions provided information concerning the average kinetic energy release, which for CO(J = 30 and 48) + H2O channel is approximately 9190 ± 1075 cm-1, and for CO(J = 9 and 20) + H2O channel it is

6028 ± 990 cm-1. The small translational energy release at low

rotational levels further supports the presence roaming behaviour.


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According to pioneering studies mentioned in the introduction, in the roaming mechanism, the fragments are rotationally colder and with little translational energy, because the required tight TS is absent. The CO products recorded with low speed at J = 8 and 20, namely the coproducts H2O are characterized by high internal energy (vibration and rotation) and low kinetic energy, are associated with the roaming channel. The presence of the tight TS, allows potential energy conversion into fragment kinetic energy. Therefore, the CO(J = 30 and 48) products recorded with high speed, or the H2O characterized by low internal energy and high kinetic energy, is probably associated with the TS channel. The absorption of an ultraviolet photon at 230 nm will promote the HCOOH molecule from the ground electronic state, S0, to the first electronically excited singlet state, S1.33 A conventional TS from trans-HCOOH has an energy barrier about 250-270 kJ/mol and the excess energy about 230 kJ/mol for 230 nm photons. The direct ab initio molecular dynamics calculation shows that the rotational quantum number of CO products obtained at 248 nm (193 nm) dissociation extends from 10 to 95 (5-105) with the averaged quantum number at 57.5 (60.8), indicating that the product CO is rotationally excited.

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These results indicated that TS path has more contribution at high rotational

level of CO. We compare the Doppler-shift speed distribution of CO between J = 30 and 48 in Fig. 5 (b), and we can see that the fast component of the speed profile of high J moves to low speed. This suggests that TS should play a relatively important role at high J, such as J = 30 and 48. The CO at high J with high kinetic energy originates from the dissociation of HCOOH on the ground state from the decay of the excited S1. 13


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Theoretical studies have shown that the calculated roaming TS has very similar energy to that of the corresponding radical dissociation channel.3, 38, 39 This indicates that the roaming species are radicals. In the case of HCOOH, two roaming dynamics could be possible in the formation of slow CO for low J’s. At the present photolysis wavelength ~230 nm, the direct OH dissociation channel on the S1 PES is the main dissociation pathway.26 The partial breaking of HCO-OH may evolve into the CO + H2O molecular channel, where OH roams around HCO and finally abstracts a hydrogen from the HCO. The OH roaming mechanism is supported by theoretical calculation of Maeda et al.

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They show that there is a S0/S1-minimum energy conical intersection (MECI) along the dissociation path of HCO + OH. The nonadiabatic transition at this MECI will cause the molecule to land on the path through the roaming TS on the S0 surface, resulting in CO + H2O after partial dissociation of HCO-OH.33 Finally, the recoiling velocity of vibrationally hot H2O and the rotationally cold CO is slow. The other roaming pathway is possibly from the partial breaking of H-COOH bond. Reaction H + COOH channel is also opened at the current photon energy, which exits though the ISC from S1 to T1 and dissociates across the barrier on the triplet PES.20,33 The “free” H atom cleaved from the parent molecule may roam around the remaining COOH and attack the OH. It should be noted, however, that the partial double bond between CO and OH increases the difficulty of H atom abstracting OH radical from CO, similar to the roaming discussion of HCOOCH3 behavior. Previous studies have found that H atom is difficult to abstract the CH3O radical from CO in the roaming dissociation of HCOOCH3 because CH3OCO appears to have partial double bond between CH3O and CO.40


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When considering the possibility of CO produced from the dissociation of hot HCO fragment, the studies with laser induced fluorescence (LIF) and REMPI detection of OH and HCO products, respectively, reveal that both are generated with little internal energy and most of the excess energy are released in the product translational energy.17-19 The studies of H atom photofragment translational spectroscopy from the photodissociation of HCOOH in the wavelength region of 240-220 nm also excludes the H products from the dissociation of hot HCO fragments.20 We notice that the velocity imaging studies of OH products at 230 nm showed that the total translational energy of OH + HCO products spreads in the range of 0 – 6000 cm-1.26 An almost zero translational energy of OH products accompanied with the hot HCO may lead to the dissociation to H + CO with HCO bond dissociation energy about 5300 cm-1. However, considering the multiple degrees of freedom of rotation and vibration, it is almost impossible that all of the internal energy of hot HCO is used to beak the H-CO chemical bond. Thus, we suggest that the CO products are mainly due to two paths from the HCOOH dissociation, the OH roaming and TS path, where the OH roaming dominates at low J’s (J ≤20 ) and the TS path contributes more at high J’s (J ≥30). The roaming and the TS paths are two independent reaction mechanisms. They result in different product speed distributions. In the dissociation of HCOOH, the overlap of speed distributions of products from the two mechanisms indicates that the roaming TS is classified as a “medium” TS, which is tighter than the relatively flat potential, loose TS, but still looser than the conventional tight TS. These discussions require theoretical support to understand the roaming dynamics in CO (v = 0, J ≤ 20) + H2O channel.


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4. CONCLUSIONS We studied the molecular channel HCOOH→CO+H2O of formic acid at 230nm by time-sliced velocity map imaging. Doppler slice images of CO for J = 9, 20, 30 and 48 have been recorded using a one-colour experiment. The fast components of CO at high rotational levels originate from dissociation via traditional TS, while the slow component of CO at low rotational levels follow roaming dissociation dynamics. The roaming pathway is mainly involved in the dissociation of CO (v = 0, J ≤ 20) + H2O channel. The partial breaking of HCO-OH leads to OH roaming around HCO and finally forming CO + H2O.

ACKNOWLEDGEMENTS The authors thank Yuxin Shen and Dong Yan for assisting with some imaging acquisitions. This work was supported by the National Natural Science Foundation of China (Nos. 21673047 and 21327901), the Shanghai Key Laboratory Foundation of Molecular Catalysis and Innovative Materials (Grant No. 16DZ2270100), Fudan University Key Laboratory senior visiting scholarship, and the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning.

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3. Quinn, M. S.; Andrews, D. U.; Nauta, K.; Jordan, M. J. T.; Kable, S. H., The energy dependence of CO(v,J) produced from H2CO via the transition state, roaming, and triple fragmentation channels. J. Chem. Phys. 2017, 147 (1), 013935. 4. Wang, X. H.; Houston, P. L.; Bowman, J. M., A new (multi-reference configuration interaction) potential energy surface for H2CO and preliminary studies of roaming. Philos T Roy Soc A 2017, 375 (2092). 5. Rubio-Lago, L.; Amaral, G. A.; Arregui, A.; Gonzalez-Vazquez, J.; Banares, L., Imaging the molecular channel in acetaldehyde photodissociation: roaming and transition state mechanisms. Phys. Chem. Chem. Phys. 2012, 14 (17), 6067-6078. 6. Rubio-Lago, L.; Amaral, G. A.; Arregui, A.; Izquierdo, J. G.; Wang, F.; Zaouris, D.; Kitsopoulos, T. N.; Banares, L., Slice imaging of the photodissociation of acetaldehyde at 248 nm. Evidence of a roaming mechanism. Phys. Chem. Chem. Phys. 2007, 9 (46), 6123-7. 7. Lee, K. L. K.; Quinn, M. S.; Maccarone, A. T.; Nauta, K.; Houston, P. L.; Reid, S. A.; Jordan, M. J. T.; Kable, S. H., Two roaming pathways in the photolysis of CH3CHO between 328 and 308 nm. Chemical Science 2014, 5 (12), 4633-4638. 8. Han, Y. C.; Shepler, B. C.; Bowman, J. M., Quasiclassical Trajectory Calculations of the Dissociation Dynamics of CH3CHO at High Energy Yield Many Products. J. Phys. Chem. Lett. 2011, 2 (14), 1715-1719. 9. Houston, P. L.; Kable, S. H., Photodissociation of acetaldehyde as a second example of the roaming mechanism. Proc Natl Acad Sci 2006, 103 (44), 16079-16082. 10. Grubb, M. P.; Warter, M. L.; Xiao, H. Y.; Maeda, S.; Morokuma, K.; North, S. W., No Straight Path: Roaming in Both Ground- and Excited-State Photolytic Channels of NO3 -> NO+O2. Science 2012, 335 (6072), 1075-1078. 11. Grubb, M. P.; Warter, M. L.; Suits, A. G.; North, S. W., Evidence of Roaming Dynamics and Multiple Channels for Molecular Elimination in NO3 Photolysis. J. Phys. Chem. Lett. 2010, 1 (16), 2455-2458. 12. Fernando, R.; Dey, A.; Broderick, B. M.; Fu, B. N.; Homayoon, Z.; Bowman, J. M.; Suits, A. G., Visible/Infrared Dissociation of NO3: Roaming in the Dark or Roaming on the Ground? J. Phys. Chem. A 2015, 119 (28), 7163-7168. 13. Kamarchik, E.; Koziol, L.; Reisler, H.; Bowman, J. M.; Krylov, A. I., Roaming Pathway Leading to Unexpected Water plus Vinyl Products in C2H4OH Dissociation. J. Phys. Chem. Lett. 2010, 1 (20), 3058-3065. 14. Lin, K. C.; Tsai, P. Y.; Chao, M. H.; Nakamura, M.; Kasai, T.; Lombardi, A.; Palazzetti, F.; Aquilanti, V., Roaming signature in photodissociation of carbonyl compounds. Int. Rev. Phys. Chem. 2018, 37 (2), 217-258. 15. Tsai, P. Y.; Hung, K. C.; Li, H. K.; Lin, K. C., Photodissociation of Propionaldehyde at 248 nm: Roaming Pathway as an Increasingly Important Role in Large Aliphatic Aldehydes. J. Phys. Chem. Lett. 2014, 5 (1), 190-195. 16. Tsai, P. Y.; Li, H. K.; Kasai, T.; Lin, K. C., Roaming as the dominant mechanism for molecular products in the photodissociation of large aliphatic aldehydes. Phys. Chem. Chem. Phys. 2015, 17 (35), 23112-23120. 17. Ebata, T.; Fujii, A.; Amano, T.; Ito, M., Photodissociation of Formic Acid - Internal State Distribution of OH Fragment. J. Phys. Chem. 1987, 91 (24), 6095-6097. 18. Ebata, T.; Amano, T.; Ito, M., Photodissociation dynamics of the S1(nπ*) state of formic acid. J. Chem. Phys. 1989, 90 (1), 112-117.


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19. Brouard, M.; Simons, J. P.; Wang, J. X., State-to-State Photodissociation Dynamics in Formic-Acid. Faraday Discuss. 1991, 91, 63-72. 20. Langford, S. R.; Batten, A. D.; Kono, M.; Ashfold, M. N. R., Near-UV photodissociation dynamics of formic acid. J Chem Soc Faraday T 1997, 93 (21), 3757-3764. 21. Shin, S. K.; Han, E. J.; Kim, H. L., Photodissociation dynamics of formic acid at 193 nm. J Photoch Photobio A 1998, 118 (2), 71-74. 22. Su, H.; He, Y.; Kong, F.; Fang, W.; Liu, R., Photodissociation of formic acid. J. Chem. Phys. 2000, 113 (5), 1891-1897. 23. Khriachtchev, L.; Macoas, E.; Pettersson, M.; Rasanen, M., Conformational memory in photodissociation of formic acid. J Am Chem Soc 2002, 124 (37), 10994-10995. 24. Lee, K. W.; Lee, K. S.; Jung, K. H.; Volpp, H. R., The 212.8-nm photodissociation of formic acid: Degenerate four-wave mixing spectroscopy of the nascent OH(X 2Πi) radicals. J. Chem. Phys. 2002, 117 (20), 9266-9274. 25. Kang, T. Y.; Kim, H. L., Photodissociation dynamics of formic acid at 206 mn. Bull. Korean Chem. Soc. 2006, 27 (12), 1997-2001. 26. Huang, C.; Zhang, C.; Yang, X., State-selected imaging studies of formic acid photodissociation dynamics. J. Chem. Phys. 2010, 132 (15), 154306. 27. Kwon, C.-H.; Choi, M.-H.; Hwang, H.-S.; Kim, H.-L., Dynamics of H Atom Production from Photodissociation of Formic Acid at 205 nm. Bull. Korean Chem. Soc. 2012, 33 (2), 728730. 28. He, H. Y.; Fang, W. H., A CASSCF/MR-Cl study toward the understanding of wavelength-dependent and geometrically memorized photodissociation of formic acid. J Am Chem Soc 2003, 125 (51), 16139-16147. 29. Borges, I.; Rocha, A. B.; Martinez-Nunez, E.; Vazquez, S., Theoretical investigations on the vibronic coupling between the electronic states S0 and S1 of formic acid including the photodissociation at 248 nm. Chem. Phys. Lett. 2005, 407 (1-3), 166-170. 30. Kurosaki, Y.; Yokoyama, K.; Teranishi, Y., Direct ab initio molecular dynamics study of the two photodissociation channels of formic acid. Chem. Phys. 2005, 308 (3), 325-334. 31. Martinez-Nunez, E.; Vazquez, S.; Granucci, G.; Persico, M.; Estevez, C. M., Photodissociation of formic acid: A trajectory surface hopping study. Chem. Phys. Lett. 2005, 412 (1-3), 35-40. 32. Martinez-Nunez, E.; Vazquez, S. A.; Borges, I.; Rocha, A. B.; Estevez, C. M.; Castillo, J. F.; Aoiz, F. J., On the conformational memory in the photodissociation of formic acid. J. Phys. Chem. A 2005, 109 (12), 2836-2839. 33. Maeda, S.; Taketsugu, T.; Morokuma, K., Automated Exploration of Photolytic Channels of HCOOH: Conformational Memory via Excited-State Roaming. J. Phys. Chem. Lett. 2012, 3 (14), 1900-1907. 34. Li, F.; Dong, C.; Chen, J.; Liu, J.; Wang, F.; Xu, X., The harpooning mechanism as evidenced in the oxidation reaction of the Al atom. Chemical Science 2018, 9, 488-494. 35. Li, F.; Ma, Y.; Liu, J.; Wang, F., Parent bending effects on nonadiabatic transition dynamics: Isotopomer-resolved imaging of photodissociation of CF3Br at two source temperatures. J. Chem. Phys. 2018, 149 (12), 124303. 36. Tjossem, P. J. H.; Smyth, K. C., Multiphoton excitation spectroscopy of the B 1Σ+ and 1 + C Σ Rydberg states of CO. J. Chem. Phys. 1989, 91 (4), 2041-2048.


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37. Riedel, J.; Dziarzhytski, S.; Kuczmann, A.; Renth, F.; Temps, F., Velocity map ion imaging of H atoms from the dissociation of HCO (A 2A'') using Doppler-free multi-photon ionization. Chem. Phys. Lett. 2005, 414 (4-6), 473-478. 38. Tsai, P. Y.; Chao, M. H.; Kasai, T.; Lin, K. C.; Lombardi, A.; Palazzetti, F.; Aquilanti, V., Roads leading to roam. Role of triple fragmentation and of conical intersections in photochemical reactions: experiments and theory on methyl formate. Phys. Chem. Chem. Phys. 2014, 16 (7), 2854-2865. 39. Bowman, J. M.; Suits, A. G., Roaming reactions: The third way. Phys Today 2011, 64 (11), 33-37. 40. Chao, M. H.; Tsai, P. Y.; Lin, K. C., Molecular elimination of methyl formate in photolysis at 234 nm: roaming vs. transition state-type mechanism. Phys. Chem. Chem. Phys. 2011, 13 (15), 7154-7161.

Fig. 1


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Figure 1 (2+1) resonance-enhanced multiphoton ionization spectrum of CO fragments through the B1Σ+ intermediate state obtained in the photodissociation of formic acid in one-colour experiment.

Fig. 2


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Figure 2 Slice images of CO (J ≈ 20) photofragments obtained at adjacent REMPI wavelengths. The direction and magnitude of the velocity of CO(J) flying relative to the direction of the laser propagation are selectively detected at specific REMPI wavelength. Due to the small energy difference of about 1.2 cm-1 between the REMPI wavelengths for the rotational states of CO in J = 19, 20 and 21, attention must be paid to the contribution of the velocity of CO from adjacent rotational states.

Fig. 3


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Figure 3 Slice images of CO recorded at (a) 230.101nm, J ≈ 9, (b)-(c) at 230.0465 and 230.0445nm, J = 30, and (d)-(f) at 229.963 nm, 229.962 nm and 229.960 nm, J = 48. At 230.0465nm, the CO(J = 29) was also recorded. The slice images of (b), (d) and (f) show the non-resonant CO background near the centre, circled in the white dashed line. Clearly, a faster component was observed in the sliced image of CO (J ≈ 30) compared to the low rotation levels of J ≈ 9 and J = 20 in Fig. 2, indicating the roaming dynamics of CO formation at low rotational states.

Fig. 4


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Figure 4 Slice images of CO recorded via Doppler scan at (a) J = 20, (b) J = 30 and (c) J = 48, respectively, after subtraction of the non-resonant central background.

Fig. 5


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Figure 5 Speed distributions of CO(J) from the corresponding slice images recorded ( a) at the respective centre wavelength of the Doppler profile (integrated in a small angular range of slice images), (b) at the tail of the Doppler profile (integrated in a small angular range of slice image), and (c) full Doppler scan. The arrow indicates the position of the maximum speed of CO(J) at the corresponding photolysis wavelength.

A brief biography of Fengyan Wang and a photograph: Fengyan Wang, the professor at the department of Chemistry, Fudan University (Shanghai), is mainly investigating molecular reaction mechanisms. In 2009, she obtained Ph.D. from the State Key Laboratory of Molecular Reaction Dynamics of the Dalian Institute of Chemical Physics,


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Chinese Academy of Sciences (supervisor: Professor Xueming Yang). Early in 2006-2008, she was supported by the EU ATLAS Marie Curie fellowship and studied at the Institute of Electronic Structure & Laser (supervisor: Professor Theofanis N. Kitsopoulos), along with a short-term study at the European Laboratory for Non-linear Spectroscopy and Vrije Universiteit. Then in 2009-2013, she conducted postdoctoral research at the Institute of Atomic and Molecular Sciences, Academia Sinica (supervisor: Professor Kopin Liu). Since 2013, she has been working at Fudan University and has established a new laser chemistry lab. By combining laser ablation, cross-molecular beam, time-sliced ion velocity imaging and various laser spectroscopy techniques, she focuses on the studies of elementary reaction mechanism, especially stateselective and stereo chemistry.

(photo of Fengyan Wang)


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Figure 1 (2+1) resonance-enhanced multiphoton ionization spectrum of CO fragments through the B1Σ+ intermediate state obtained in the photodissociation of formic acid in one-colour experiment. 338x190mm (300 x 300 DPI)



Figure 2 Slice images of CO (J ≈ 20) photofragments obtained at adjacent REMPI wavelengths. The direction and magnitude of the velocity of CO(J) flying relative to the direction of the laser propagation are selectively detected at specific REMPI wavelength. Due to the small energy difference of about 1.2 cm-1 between the REMPI wavelengths for the rotational states of CO in J = 19, 20 and 21, attention must be paid to the contribution of the velocity of CO from adjacent rotational states.


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Figure 3 Slice images of CO recorded at (a) 230.101nm, J ≈ 9, (b)-(c) at 230.0465 and 230.0445nm, J = 30, and (d)-(f) at 229.963 nm, 229.962 nm and 229.960 nm, J = 48. At 230.0465nm, the CO(J = 29) was also recorded. The slice images of (b), (d) and (f) show the non-resonant CO background near the centre, circled in the white dashed line. Clearly, a faster component was observed in the sliced image of CO (J ≈ 30) compared to the low rotation levels of J ≈ 9 and J = 20 in Fig. 2, indicating the roaming dynamics of CO formation at low rotational states.



Figure 4 Slice images of CO recorded via Doppler scan at (a) J = 20, (b) J = 30 and (c) J = 48, respectively, after subtraction of the non-resonant central background.


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Figure 5 Speed distributions of CO(J) from the corresponding slice images recorded ( a) at the respective centre wavelength of the Doppler profile (integrated in a small angular range of slice images), (b) at the tail of the Doppler profile (integrated in a small angular range of slice image), and (c) full Doppler scan. The arrow indicates the position of the maximum speed of CO(J) at the corresponding photolysis wavelength.


Roaming Dynamics in the Photodissociation of Formic Acid at 230 nm

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