tokens: those observed in Ar, allowing a clear assignment. The most striking observation is that those bands are very weak in comparison to other bands growing in this region (Figs. 2 and 3 ). In the 23002000 cm zone (Fig. 2 ), new bands are formed during the irradiation. Among them, a band at 2079.1 cm can clearly be assigned to the HCN molecule ( Milligan Jacox 1967 ). This assignment is confirmed in other regions of the spectrum where characteristic bands of HCN are observed especially at 3306.4 cm (Fig. 3 ) and 723.5 cm. At longer irradiation times, the HCN dimer is formed with a band at 791.9 cm ( King Nixon 2003 ) and oligomers with a band at 809.3 cm (cf. Table 2 , for detailed assignments). Bands at 3622.2 and 2028.6 cm are characteristic of the HCN photoisomerisation producing the isonitrile HNC. After 75 min of irradiation, the CN radical ( Jacox Thompson 2007 ) is detected with its characteristic band at 2050.8 cm. One additional peak is also detected at 1337.5 cm, which is tentatively assigned to the radical HCN that was already observed in an Ar matrix ( Milligan Jacox 1967 ). The first interpretation of these results is that the CN radical formed by the CN photodissociation subsequently reacts by abstraction of one H atom of H molecules to form the hydrogenated compound. To be sure that the products are formed from the CN radicals reacting with H and not by CN radical reacting with H atoms, we developed a strategy changing the wavelength of the irradiation in addition to using a D matrix. Fig. 2 IR spectra of CN in a H matrix in the range 23002000 cm at different irradiation times using a hydrogen lamp and assignment of the new species. a) deposition; b) 5 min; c) 15 min; d) 35 min; e) 75 min; f) 135 min and g) 195 min of irradiation. D = Dimers of HCN, M = Monomer of HCN, P = Polymers of HCN. Fig. 3 IR spectra of CN in a H matrix in the range 35002800 cm at different irradiation times using a hydrogen lamp and assignment of the new species. a) deposition; b) 5 min; c) 15 min; d) 35 min; e) 75 min; f) 135 min and g) 195 min of irradiation. D = Dimers of HCN, M = Monomer of HCN, P = Polymers of HCN. 3.4. C<inline-formula specific-use="simple-math"><sub>2</sub></inline-formula>N<inline-formula specific-use="simple-math"><sub>2</sub></inline-formula> VUV irradiation in a H<inline-formula specific-use="simple-math"><sub>2</sub></inline-formula> matrix using a sapphire window (<inline-formula specific-use="simple-math"><italic>λ</italic> > 147</inline-formula> nm) and an Ar lamp (<inline-formula specific-use="simple-math"><italic>λ</italic></inline-formula> > 160 nm) The edge of the absorption band of H overlaps with the 121 nm radiation ( Bunch et al. 1958 ) and therefore we have to take into account that an excited state or photodissociation of H could be involved in the previous formation of HCN. To verify that the observed reactivity is exclusively due to the cleavage of cyanogen, we changed the MgF window of our system with a sapphire
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