VUV photochemistry of the H2O CO complex in noble-gas matrices: formation of the OH CO complex and the HOCO radical

Vacuum ultraviolet (VUV, 130–170 nm) photochemistry of the H2O CO complex is studied by matrixisolation infrared spectroscopy. The H2O CO complexes in Ne, Ar, Kr, and Xe matrices are generated by ultraviolet (UV, 193 and 250 nm) photolysis of formic acid (HCOOH). VUV photolysis of the H2O CO complexes is found to lead to the formation of the OH CO radical–molecule complexes and transHOCO radicals. It is shown that the matrix material, local matrix morphology, and possibly the H2O CO complex geometry strongly affect the VUV photolysis pathways. The intrinsic reactivity of the matrixisolated OH CO complex resulting in the formation of trans-HOCO is directly demonstrated for the first time. This reaction occurs in Ar, Kr, and Xe matrices upon annealing above 25 K and may proceed over the barrier. The case of a Ne matrix is very special because the formation of trans-HOCO from the OH CO complex is observed even at the lowest experimental temperature (4.5 K), which is in sharp contrast to the other matrices. It follows that quantum tunneling is probably involved in this process in the Ne matrix at such a low temperature. Infrared light also promotes this reaction in the Ne matrix at 4.5 K, which is not the case in the other matrices. The last findings show the effect of the environment on the tunneling and infrared-induced rates of this fundamental chemical reaction.


Introduction
Non-covalent interactions play an important role in a great variety of chemical processes. Light-induced chemical transformations in weakly-bound intermolecular complexes represent an intriguing field of supramolecular photochemistry with a remarkable impact on life science, nanotechnology, and chemical synthesis. 1,2 The matrix-isolation technique has been extensively used for the investigation of non-covalent interactions, particularly, involving radicals and other unstable species. [3][4][5][6][7][8][9][10][11][12][13] Recent studies have demonstrated an effective approach for the preparation of matrix-isolated complexes in high concentrations using the photolysis of appropriate molecular precursors (see ref. 9 for a review). For example, UV photolysis of formic acid (HCOOH, FA) leads to the formation of the H 2 OÁ Á ÁCO complexes. 14 Furthermore, one can use matrix isolation to investigate the photochemistry (or, in a wider context, radiation-driven chemistry) of intermolecular complexes. The photoinduced transformations of some complexes have been studied previously under matrix-isolation conditions. [15][16][17][18][19][20][21] In general, such reactions can lead to the assembly of new species or ''cold synthesis'' under the conditions of frozen molecular mobility, which may be particularly significant to model and understand the processes in interstellar and cometary ices.
Water and carbon monoxide are the two most abundant molecules in interstellar ices, 22 and these species also play an important role in the atmospheric chemistry. The intermolecular complex H 2 OÁ Á ÁCO has been studied previously both theoretically and experimentally. 14, [23][24][25][26][27][28][29][30][31][32][33][34] The reaction of this complex with an H atom produces the radical-molecule complex HCOÁ Á ÁH 2 O. 35 Another related radical-molecule complex, OHÁ Á ÁCO, is expected to be the key primary product of the photoinduced transformation of H 2 OÁ Á ÁCO. The OHÁ Á ÁCO complex has been extensively examined by ab initio calculations as an intermediate in the OH + CO -CO 2 + H reaction, which is significant in combustion and atmospheric processes. [36][37][38][39][40][41][42][43] Lester et al. reported an experimental detection of the OHÁ Á ÁCO complex in the gas phase having the OH stretching overtone at 6941.8 cm À1 (shifted by À29.6 cm À1 with respect to the OH monomer). 37 The electronic excitation spectrum and the intermolecular vibrational frequencies of this complex have also been characterized experimentally. 44 Very recently, Brice et al. have reported the OH fundamental frequency of the OHÁ Á ÁCO complex in He droplets at 3551 cm À1 (shifted by À17 cm À1 with respect to the OH monomer). 45 Nevertheless, full experimental characterization of the fundamental IR absorptions of the OHÁ Á ÁCO complex is still lacking. † Another species that is relevant to this system is the carboxyl radical (HOCO) that is separated from the OHÁ Á ÁCO complex by a low energy barrier (a few hundred cm À1 ). [37][38][39][40]42 HOCO is an important intermediate in combustion and atmospheric chemistry and it is also assumed to be involved in the formation of prebiotic molecules in the interstellar medium. [46][47][48][49][50] This radical has two conformers, the trans form being lower in energy than the cis form by about 500 cm À1 . It has been shown that trans-HOCO can be produced in low-temperature matrices upon photolysis or radiolysis of some CO-or CO 2 -containing intermolecular complexes. 19a,21,51 This species was also detected in the VUV-and electron-irradiated H 2 O/CO solid mixtures. 52 The rotational spectra of the two conformers of HOCO and their deuterated analogs in the gas phase were obtained by FTMW spectroscopy and the millimeter-wave double resonance technique. 53 The aim of the present work is to study vacuum ultraviolet (VUV) photochemistry of the H 2 OÁ Á ÁCO complex under matrixisolation conditions and identify the OHÁ Á ÁCO radical-molecule complex. This study is also related to the photochemical preparation of the carboxyl radical HOCO. UV photolysis of matrixisolated FA is used to produce a sufficient concentration of the H 2 OÁ Á ÁCO complexes in noble-gas matrices. 14

Experimental details
The experiments were carried out using FA/Ng mixtures (Ng = Ne, Ar, Kr, and Xe) with typical ratios of 1/1000-1/2000. FA (Merck, 498%) was degassed by several freeze-pump-thaw cycles. Ne (99.999%), Ar (99.9999%), Kr (99.999%), and Xe (99.999%) were used as purchased (Linde). The gas FA/Ng mixtures were made in a glass bulb by using standard manometric procedures. Since FA is easily adsorbed on glass surfaces, the bulb was passivated with FA vapors by fill-keep-evacuate cycles prior to the mixture preparation. The FA/Ng matrices were deposited onto a CsI substrate held at 4, 15, 20, and 30 K for Ne, Ar, Kr, and Xe matrices, respectively, in a closed-cycle helium cryostat (RDK-408D2, SHI). The FA/Ng matrices (Ng = Ne, Ar, and Kr) were photolyzed with an excimer laser (MSX-250, MPB) operating at 193 nm (B10 mJ cm À2 ) to produce the H 2 OÁ Á ÁCO complexes. Since 193 nm photons decompose water in a Xe matrix, 54 in this case we used 250 nm radiation (B5 mJ cm À2 ) of an optical parametric oscillator (OPO, Sunlite, Continuum). After UV photodecomposition of FA, the matrix was exposed to VUV light (130-170 nm) of a Kr lamp (Opthos). During VUV photolysis, the space between the lamp and the cryostat optical window (MgF 2 ) was flushed with argon. Reference experiments were also made with FA/Ng matrices initially photolyzed with VUV radiation of a Kr lamp, i.e. without preliminary UV photolysis. Selective excitation in the IR region was performed by narrowband light of an OPO (LaserVision). The IR absorption spectra in the 4000-600 cm À1 range were measured at 4.5 K using an FTIR spectrometer (Vertex 80, Bruker) with 1 cm À1 resolution and 500 scans.

Results
In these experiments, the H 2 OÁ Á ÁCO complex was generated in noble-gas matrices by photodecomposition of FA. The deposited FA/Ng matrices (Ng = Ne, Ar, Kr, and Xe) contained mostly FA monomers (in the more stable trans form) 14 with small amounts of FA dimers (tt1 and tt2) 55,56 and trans-FAÁ Á ÁH 2 O complexes. 56,57 The obtained spectra of trans-FA are in full agreement with the previous data. UV irradiation of the deposited matrices (B5000 pulses at 193 nm for Ng = Ne, Ar, and Kr and B20 000 pulses at 250 nm for Ng = Xe) resulted in efficient decomposition of FA (typically 470%). The difference IR spectra showing the result of UV photolysis in different matrices are presented in Fig. 1. The increase of the absorbance of H 2 OÁ Á ÁCO bands correlates with the FA photodecomposition. The fundamental frequencies of the H 2 OÁ Á ÁCO complex isolated in noble gas matrices are given in Table 1. The spectra of the H 2 OÁ Á ÁCO complex observed in our experiments in Ar, Kr, and Xe matrices are in good agreement with previous works, 14,27c however, some additional weak features of the perturbed H 2 O fundamentals are also detected. These weaker bands correlate with the other H 2 OÁ Á ÁCO absorption bands. We suppose that the absence of these features in the earlier work 14 may be due to higher temperatures in their experiments (15 K). The IR absorptions of the H 2 OÁ Á ÁCO complex in the Ne matrix have been previously reported only for n 2 (H 2 O) and n(CO). 58 In our work, we also observed the n 1 and n 3 bands of water in this complex.
The H 2 OÁ Á ÁCO complex can have two different structures, HOHÁ Á ÁCO and HOHÁ Á ÁOC, in which a water hydrogen interacts with different atoms of carbon monoxide (C and O, respectively). These two structures can be distinguished spectroscopically by the CO stretching shift with respect to CO monomer. The computationally more stable HOHÁ Á ÁCO complex has a blue shift of this mode, whereas the HOHÁ Á ÁOC complex shows a red shift. 14,27 In full agreement with the previous results, 14 we observed a matrix-specific ratio of these complexes produced by UV photolysis of FA. In fact, only the HOHÁ Á ÁCO structure is observed in Ne and Ar matrices; both HOHÁ Á ÁCO and HOHÁ Á ÁOC structures are found in a Kr matrix, and the HOHÁ Á ÁOC form dominates in a Xe matrix. It is worth noting that in Kr and Xe matrices, the bands of water in the H 2 OÁ Á ÁCO complex are quite structured, which is most probably due to several matrix sites with overlapping absorptions. This fact prevents us from definite assignment of these bands to the two complex structures (HOHÁ Á ÁCO and HOHÁ Á ÁOC). In addition to the H 2 OÁ Á ÁCO complex, CO 2 (possibly complexed with H 2 ) and monomeric CO are also produced by UV photolysis of FA. The relative yield of CO (both complexed and monomeric) with respect to CO 2 decreases with the increase of the matrix dielectric constant, which is discussed elsewhere. 14 The effect of Kr-lamp radiation (130-170 nm) on matrices containing the H 2 OÁ Á ÁCO complexes is shown in Fig. 2. This irradiation resulted in gradual bleaching of the H 2 OÁ Á ÁCO bands, an increase of the band of the CO monomer (2140.9, 2138.5, 2135.5 and 2133.0 cm À1 in Ne, Ar, Kr, and Xe, respectively) 21 35,56 were also detected. In addition, a new absorber with bands at 3546.5 and 2159.8 cm À1 in a Ne matrix, at 3526.5 and 2152.5 cm À1 in an Ar matrix, and at 3521.9 and 2147.9 cm À1 in a Kr matrix was observed. These bands are assigned in this work to the OHÁ Á ÁCO complex as discussed below. In this series of experiments, no candidates for the OHÁ Á ÁCO absorptions were found after VUV photolysis in the Xe matrix even though the trans-HOCO bands were quite evident. It should be stressed that trans-HOCO and the OHÁ Á ÁCO complex were not observed during the initial UV photolysis. It is also worth noting that no evidence for the stabilization of the higher energy cis-HOCO conformer has been found in these studies although it was reported previously in N 2 and CO matrices as well as in solid H 2 O-CO mixtures. 19a,52 To our knowledge, cis-HOCO has never been observed in noblegas matrices.
The formation of trans-HOCO and OHÁ Á ÁCO tends to saturate for substantial consumption of H 2 OÁ Á ÁCO, whereas the production of CO 2 continues. This behavior can be interpreted as an interplay between primary (production of OHÁ Á ÁCO and trans-HOCO) and secondary (production of CO 2 ) channels of VUV photolysis of the H 2 OÁ Á ÁCO complex. The direct photoproduction of CO 2 from H 2 OÁ Á ÁCO is also possible.
After the first (UV) photolysis, the matrices contain some remaining amounts of FA. As seen in Fig. 2, FA is partially In a separate series of experiments, we annealed the matrices after UV photolysis (prior to VUV photolysis). The annealing temperatures were 25, 35, and 40 K for Ar, Kr, and Xe matrices respectively. Ne matrices were not treated in this way since they are stable only below 12 K. This annealing led to some changes in the matrix sites of the H 2 OÁ Á ÁCO complex. An example of the Ar matrix is shown in Fig. 3. Based on the annealing-induced behavior, we distinguish the IR absorptions of the H 2 OÁ Á ÁCO complex in unrelaxed (decreasing upon annealing) and relaxed (increasing upon annealing) matrix   sites irrespective of the geometry of the complex (see Table 1). The absorptions attributed to the unrelaxed sites are broader and/or more structured compared to the ones of the relaxed sites. It should be noted that the complete conversion to the relaxed matrix site (as in the case of the Ar matrix) was not achieved in the Kr matrix. In the Xe matrix, the weak band at 2141.0 cm À1 slightly increased in intensity, whereas no candidates for the relaxed site were found in the water stretching region. To remind, the yield of the H 2 OÁ Á ÁCO complex in the Xe matrix was lower than in the other matrices. The observed effect of annealing on the H 2 OÁ Á ÁCO bands is in good agreement with the previous observations. 14a We have also found that annealing of the UV-irradiated FA/Ng matrices leads to the formation of HCO Annealing of the matrices prior to VUV photolysis significantly affects the branching ratio of the photoproducts. In fact, the ratio of the OHÁ Á ÁCO and trans-HOCO amounts markedly increased in the annealed matrices. Fig. 4 illustrates this result for the Ar matrix. A similar trend was observed in the Kr matrix. In the Xe matrix annealed after UV photolysis, VUV photolysis produced a weak band at 2145.0 cm À1 , which was attributed to the CO absorption of the OHÁ Á ÁCO complex; however, no band of this complex in the OH stretching region was found in this experiment.
Annealing of the matrices after VUV photolysis results in a decay of the OHÁ Á ÁCO complex as illustrated for the Ar matrix in Fig. 5a. In Ar and Kr matrices, the OHÁ Á ÁCO bands almost disappear after annealing at 30-35 K. Fig. 5b shows the effect of annealing on the OHÁ Á ÁCO decay and the formation of trans-HOCO and HCOÁ Á ÁH 2 O in the Ar matrix. First, there is no correlation between the formation of HCOÁ Á ÁH 2 O, which is due to the H + COÁ Á ÁH 2 O reaction 35 and the OHÁ Á ÁCO decay. This fact evidences that the main decomposition channel of OHÁ Á ÁCO is not connected with mobile H atoms. Second, there is a good overall correlation between the decomposition of OHÁ Á ÁCO and the formation of trans-HOCO, particularly above 25 K, which features the OHÁ Á ÁCOtrans-HOCO reaction. It should be understood that the strict correlation between these processes is difficult to expect due to a number of additional channels. For example, some of the OH and CO fragments can be shortly separated in the matrix after photolysis and recombine upon annealing. Agglomeration of species upon annealing is another complicating factor. The probable reaction of the OHÁ Á ÁCO complex with H atoms can also change the temperature dependences. In this respect, we can mention that some increase of the amount of the H 2 OÁ Á ÁCO complex is systematically observed after annealing. In the Xe matrix, the amount of OHÁ Á ÁCO is very small, and it is difficult to study this process in detail. However, this annealing allowed us to detect the OH fundamental of the OHÁ Á ÁCO complex in the Xe matrix (at 3520.0 cm À1 ), which was otherwise masked by the absorption of residual FA (see Fig. 6). In Kr and Xe matrices, annealing at 35 and 45 K, respectively, also resulted in a slight decrease of the trans-HOCO bands (see the lower trace in Fig. 6). A similar behavior was observed previously and may indicate the reaction of trans-HOCO with mobile H atoms. 56 As to the case of the Ne matrix, a slow decay (on a timescale of more than 10 h) of the OHÁ Á ÁCO complex accompanied by the formation of trans-HOCO was observed even in the dark at 4.5 K (the lowest temperature of our experiment) (see Fig. 7). It is worth mentioning that no other products of OHÁ Á ÁCO decay (e.g. CO 2 ) were found in these experiments, except for small amounts of the H 2 OÁ Á ÁCO complex. According to our kinetics measurements, the decay of the OHÁ Á ÁCO complex in the dark is not a single exponential but slows down in time, which is common for solid-state reactions. 65 The decay of the OHÁ Á ÁCO complex is accelerated by temperature; however, the narrow temperature range of stability of the Ne matrix and the complicated decay curve prevented further studies on this matter. In addition, it was found that this process was accelerated by broadband IR light of the globar and selective excitation of the OH mode by narrowband radiation of OPO (3546.5 cm À1 ). IR light seemed to decrease the trans-HOCO yield (compared to the process in the dark), but additional channels were not observed. It should be stressed that no dark OHÁ Á ÁCOtrans-HOCO reaction was detected in other matrices at 4.5 K as well as there was no efficient effect of IR light.
Finally, we examined the photostability of the OHÁ Á ÁCO and trans-HOCO species in the Kr matrix. It was found that B250 pulses at 193 nm decomposed trans-HOCO almost completely, whereas the OHÁ Á ÁCO bands remained virtually unchanged. The 193 nm photolysis of trans-HOCO yielded CO 2 , but the amount of CO monomer did not change.

Discussion
Light-induced decomposition of water is known to occur via the H 2 O -H + OH channel and the energy of Kr-lamp photons is enough to dissociate water in matrices. 66 Therefore, one may expect the formation of the OHÁ Á ÁCO complex upon photolysis of the H 2 OÁ Á ÁCO complex. In the present work, we observed two bands, correlating with the VUV-induced decomposition of H 2 OÁ Á ÁCO, which are assigned to the fundamental vibrations of the OHÁ Á ÁCO complex. The OH and CO absorption bands appear at 3546.5 (Ne), 3526.5 (Ar), 3521.9 (Kr), and 3520.0 cm À1 (Xe) and at 2159.8 (Ne), 2152.5 (Ar), 2147.9 (Kr), and 2145.0 cm À1 (Xe), respectively.
The available calculations predict two structures for this complex, which can be represented as OHÁ Á ÁCO and OHÁ Á ÁOC (both linear), the OHÁ Á ÁCO structure being energetically more favorable (by 140-180 cm À1 ). [37][38][39]42,43 According to the calculations, the experimentally observed complex has the OHÁ Á ÁCO structure, in which the hydrogen atom of OH interacts with the carbon atom of CO. Table 3 shows the experimental and calculated shifts of the OHÁ Á ÁCO bands with respect to the OH and CO monomers. The experimental values are in good agreement with the theory. We did not observe any candidates for the less stable OHÁ Á ÁOC structure. This species should have a blue-shifted OH absorption and a red-shifted CO one, i.e. the directions of the shifts are opposite in comparison with those of OHÁ Á ÁCO. 38,43,45 Thus, it seems that the less stable OHÁ Á ÁOC structure is not stabilized under our experimental conditions. It is worth noting that this result is in agreement with the studies in the gas-phase and He droplets, where only the OHÁ Á ÁCO structure was detected. 37,45   The decrease of the complexation-induced shift with the increase of the matrix dielectric constant is an interesting observation (see Table 3). This trend is presumably due to a weaker matrix effect on vibrations of the complex units compared to the monomers. It was previously discussed that the complexationinduced shift is worth counting from the monomer in a vacuum rather than from that in the matrix. 21 A Ne matrix is presumably the best approximation of a vacuum among the practical noble gases and the very good agreement of the experiment in the Ne matrix with calculations of ref. 43 is remarkable.
trans-HOCO is another product observed upon VUV photolysis of the H 2 OÁ Á ÁCO complex. The previous matrix-isolation studies have shown that this radical can be produced in different ways. First, it is known that trans-HOCO can be formed in low-temperature matrices via the OH + COtrans-HOCO reaction. In this case, the CO molecule should be in close proximity to the species that served as a precursor of the OH radical. This reaction pathway has been demonstrated in a number of experimental works, where trans-HOCO was generated upon VUV photolysis of H 2 O in solid CO, 52a UV photolysis of the COÁ Á ÁH 2 O 2 complex in noble-gas matrices, 21 near-UV photolysis of the COÁ Á ÁHONO complex in a N 2 matrix, 19a and X-ray radiolysis of the H 2 OÁ Á ÁCO 2 complex in noble-gas matrices. 51 Upon X-ray radiolysis of matrix-isolated FA, the direct dissociation channel HCOOH -HOCO + H occurs, 56 which was not found previously in UV photolysis. In the present work, trans-HOCO was efficiently produced in the ''two-color'' photochemical experiments with FA/Ng matrices, i.e. when UV photolysis of the matrix was followed by VUV photolysis.
The previous works on UV photolysis of matrix-isolated FA showed only two primary channels, HCOOH -H 2 O + CO and HCOOH -H 2 + CO 2 . 14 The present results suggest the existence of additional channels. First, UV photolysis of matrix-isolated FA efficiently produces H atoms, which is evidenced by annealinginduced reactions. It follows that some radical products are formed under these conditions. Second, trans-HOCO is rapidly decomposed by UV radiation most probably yielding CO 2 and H atoms. It is possible that the direct dissociation of FA to trans-HOCO and the H atom occurs upon UV photolysis, similarly to X-ray radiolysis, 56 but trans-HOCO is not observed because of its very efficient photodecomposition. In addition, we cannot exclude the production of H atoms via the photodecomposition of HCO. Indeed, the reaction HCOOH -HCO + OH is a main channel in the gas-phase photolysis of FA. 67 In this situation, the photodecomposition of HCO and possibly HOCO may be responsible for the production of the CO monomer observed in significant amounts in Kr and Xe matrices (see Fig. 1). Thus, the UV photochemistry of matrix-isolated FA probably deserves further studies.
In our experiments with VUV irradiation, trans-HOCO is mainly formed from the H 2 OÁ Á ÁCO complex although the direct dissociation channel HCOOH -HOCO + H is also possible. The present results cannot directly prove whether the H 2 OÁ Á ÁCOtrans-HOCO photoreaction (VUV) is a single-step process or it occurs via the OHÁ Á ÁCO intermediate. It is difficult to distinguish these two channels based on the kinetic measurements because the VUV-induced degradation of both trans-HOCO and OHÁ Á ÁCO (yielding CO 2 ) significantly complicates this analysis. Nevertheless, the possibility of the two-step mechanism H 2 OÁ Á ÁCO -OHÁ Á ÁCOtrans-HOCO is indirectly supported by the observed transformation of OHÁ Á ÁCO to trans-HOCO, and this process is discussed in more detail below.
The gas-phase reaction between OH and CO leading to trans-HOCO has been suggested to occur via the OHÁ Á ÁCO complex. 37,68,69 Our observations provide valuable information about the reactivity of the OHÁ Á ÁCO complex. The annealinginduced decay of OHÁ Á ÁCO occurring in Ar and Kr matrices at B30 K correlates with the formation of trans-HOCO. As mentioned above, the lack of a strict correlation between these processes observed in the Ar matrix below 25 K (see Fig. 5b) features additional reactions. As suggested previously, 51 the recombination of shortly-separated (non-complexed) pairs OH + COtrans-HOCO may be responsible for the formation of trans-HOCO at temperatures between 10 and 25 K. The short-range recombination with H atoms recovering the H 2 OÁ Á ÁCO complex can contribute to the decay of OHÁ Á ÁCO at B15 K. In the Ne matrix, the OHÁ Á ÁCO decay in the dark is also accompanied by the formation of trans-HOCO. Thus, we conclude that trans-HOCO is the main product of decomposition of the OHÁ Á ÁCO complex under our experimental conditions and an additional minor channel may be the OHÁ Á ÁCO + H -H 2 OÁ Á ÁCO reaction. The annealing-induced reaction OHÁ Á ÁCOtrans-HOCO in Ng matrices (Ng = Ar, Kr, and Xe) can be an over-barrier process. Indeed, the Arrhenius equation yields the characteristic reaction time of ca. 10 s at 30 K and 2 Â 10 21 years at 10 K, for the reaction barrier of 665 cm À1 and the pre-exponential factor of 7.4 Â 10 12 s À1 . ‡ In the Ne matrix, the OHÁ Á ÁCO decay and the formation of trans-HOCO occur in the dark even at 4.5 K. This observation can be considered as a signature of quantum tunneling because the over-barrier reaction at such a low temperature can be excluded. The OHÁ Á ÁCOtrans-HOCO reaction induced by IR light also takes place in the Ne matrix. It is interesting that in other matrices, the OHÁ Á ÁCOtrans-HOCO process was not found at 4.5 K. Concerning the possible tunneling mechanism, this difference may be connected with the details of the matrix environment (solvation), in agreement with the known observations. 70 It is more difficult to understand the lacking efficiency of the IR-induced process in other noble-gas matrices. We can speculate that the vibrational energy relaxation of OHÁ Á ÁCO is faster for heavier matrix atoms, and the required reorganization of the complex does not occur. Finally, we discuss the effects of the matrix material (see Fig. 2) and matrix annealing (see Fig. 4) on the branching ratio of the OHÁ Á ÁCO and trans-HOCO channels of VUV photolysis of the H 2 OÁ Á ÁCO complex. First, this ratio sufficiently decreases in the Kr matrix as compared to Ar and Ne matrices. In the Xe matrix, the OHÁ Á ÁCO formation is very inefficient. Second, the relative yield of the OHÁ Á ÁCO production can be sufficiently increased by annealing of the matrices after initial UV photolysis (prior to VUV-photolysis). These trends qualitatively correlate with the matrix site splitting of the H 2 OÁ Á ÁCO absorptions. The ratio between the H 2 OÁ Á ÁCO amounts in the relaxed and unrelaxed matrix sites after UV photolysis of FA decreases from Ne to Xe. We believe that the bands of H 2 OÁ Á ÁCO in the Ne matrix correspond to the relaxed matrix site because they are narrow and unstructured. Matrix annealing stimulates conversion of the unrelaxed site to the relaxed one. (In general, annealing leads to relaxation of matrix morphology to a lower energy.) These correlations seem to show that VUV photolysis of the H 2 OÁ Á ÁCO complex in an unrelaxed environment produces mainly trans-HOCO. In contrast, a more efficient formation of the OHÁ Á ÁCO complex occurs in a relaxed environment, e.g., after annealing of the matrix. The exact structure and energetics of these matrix sites are unknown.
In addition to matrix morphology, the geometry of the parent H 2 OÁ Á ÁCO complex can affect its VUV photoproducts. It is possible that the photochemical channels are different for the HOHÁ Á ÁCO and HOHÁ Á ÁOC structures. For example, the low yield of the OHÁ Á ÁCO complex in the Xe matrix may be connected with the dominating HOHÁ Á ÁOC structure of the precursor. However, to examine this effect, one needs to obtain matrices with different structures of the H 2 OÁ Á ÁCO complex (HOHÁ Á ÁCO only and HOHÁ Á ÁOC only). In the present work, we failed to find experimental conditions providing these matrices and this question remains unanswered.

Conclusions
In the present work, we have studied VUV photolysis (130-170 nm) of the H 2 OÁ Á ÁCO complexes in noble-gas matrices (Ne, Ar, Kr, and Xe). The H 2 OÁ Á ÁCO complexes were generated by UV photolysis (193 and 250 nm) of formic acid (FA) in noble-gas matrices. In agreement with the previous observations, 14 the matrix material has a significant effect on both the yield and the geometry of the H 2 OÁ Á ÁCO complex. As a new finding, UV photolysis of FA is observed to produce considerable amounts of H atoms. The formation of H atoms possibly includes the photolysis of trans-HOCO and/or HCO directly produced from FA. This observation shows that UV photochemistry of FA in noble-gas matrices is not fully understood.
VUV photolysis of the H 2 OÁ Á ÁCO complex in noble-gas matrices leads to the formation of the OHÁ Á ÁCO radical-molecule complex, in which the hydrogen atom of OH interacts with the carbon atom of CO. All fundamental absorptions of this complex were found in the four matrices. The red shifts of À26.5, À21.5, À16.7, and À11.4 cm À1 (with respect to OH monomer) were observed for the OH stretching mode in Ne, Ar, Kr and Xe matrices, respectively, whereas the blue shifts of +18.9, +14.5, +12.4 and +12.0 cm À1 were found for the corresponding CO stretching vibrations. The obtained data are in good agreement with the calculations reported previously. 38,43,45 No evidence for the stabilization of the less stable OHÁ Á ÁOC structure was found. The formation of the OHÁ Á ÁCO complex is quite efficient in Ne and Ar matrices, and it is hardly visible in the Xe matrix. The yield of the OHÁ Á ÁCO complex increased if the matrices containing the H 2 OÁ Á ÁCO complexes were subjected to annealing prior to VUV photolysis. These observations suggest that the local matrix morphology and possibly also the H 2 OÁ Á ÁCO complex geometry affect the photolysis pathways.
The trans-HOCO radicals are efficiently formed by VUV photolysis of the matrix-isolated H 2 OÁ Á ÁCO complexes. Several new assignments are suggested for this species. We have to note that the multi-step production of trans-HOCO from FA via the H 2 OÁ Á ÁCO complex is a promising approach for further matrix-isolation studies on this important species. It is worth noting that neither the OHÁ Á ÁCO complexes nor the trans-HOCO radicals appear after UV photolysis of FA in the noble-gas matrices; thus, the VUV photolysis has a clear advantage in this case.
Finally, the intrinsic reactivity of the matrix-isolated OHÁ Á ÁCO complex resulting in the formation of trans-HOCO is directly demonstrated. This reaction was observed in Ar and Kr matrices upon annealing at 30-35 K. These results confirm that the OHÁ Á ÁCO complex can act as an intermediate in the OH + CO -HOCO reaction. The case of the Ne matrix is special because the formation of trans-HOCO from the OHÁ Á ÁCO complex occurs in the dark even at the lowest experimental temperature (4.5 K), which is in contrast to other matrices. It is possible that quantum tunneling is involved in this process because the over-barrier reaction is improbable at such a low temperature. If it is true, this finding represents a strong matrix effect on the tunneling rate. IR light activates this reaction in the Ne matrix at 4.5 K, and again, this effect is not certainly found in the other matrices, which is not fully understood. The Ne matrix is the closest approximation of the gas phase among the noble-gas solids; thus, the tunneling and IR-induced reactions found in the Ne matrix may also occur in the gas phase.