3. Spectroscopic results
In order to retain the linearity of the energy scale we use wavenumbers (cm-1) rather than wavelengths to present the results. The NIR emission spectra recorded during excitation of CO at 63938.6 cm-1 into the A(0) X(0) transition is shown in Fig. 1. The emission spectrum between 16000 and 11000 cm-1 was recorded with the photomultiplier and the spectrum between 12500 and 6650 cm-1 was recorded with the photodiode. Due to different spectral responses of the photomultiplier and the photodiode, we did not attempt to normalise the spectra with each other. The excitation spectra measured by monitoring the emission at 14050 and 12311 cm-1 have been presented in an earlier publication (Gudipati 1997) and the excitation spectra recorded by monitoring the other strong emission bands at 10588, 9506, 8897 and 7770 cm-1 are collected in Fig. 2. Energy levels of the triplet states and the singlet A =0) state with respect to the zero vibrational level of the a-state are sketched in Fig. 3. For comparison gas-phase energies derived from the spectroscopic constants given in the paper of Bahrdt et al. (1987) are also included in Fig. 3.
It is obvious from the spectra presented in Figs. 1 & 2 that the emission results from CO. The emission bands could not be assigned as being due to a single upper electronic state. On the other hand, the gas-phase NIR emission bands are exclusively due to the a a transition (Effantin et al. 1982, Amoit & Islami 1986).
In order to analyse the present spectroscopic data and compare it with the date reported in the literature, it is necessary to consider the following aspects. In the present work isolated CO molecules are excited directly with electromagnetic radiation. On the other hand, excitation of CO in the gas-phase was achieved in a plasma generated through microwave or electric discharge. Under these experimental conditions collisions as well as recombination of C and O atoms generate the excited CO molecules. In spite of these differences, the spectroscopic data derived from the gas-phase experiments (Effantin et al. 1982, Amoit & Islami 1986) is accurate and reliable. For example, the a (2) a(0) transition at 9278.12 cm-1 measured by Effantin et al. (1982) is close to the value of 9282.28 cm-1 for the a (2) a(0) transition derived from the spectroscopic constants given by Huber & Herzberg (1979). Hence a comparison of the spectroscopic data from the present work with that of the gas-phase studies is justified.
The second aspect that needs to be considered is the dipole moment of CO in different electronic states. The dipole moments () of the first five electronic states of CO are discussed in detail by Lynch et al. (1982), which are summarised here. A positive value of indicates positive charge on C and negative charge on O (C O ). These are: in the X state, = -0.122 D (exp.) & -0.331 D (theory); in the a state, = -2.487 D (theory); in the d state, = -2.314 D (theory); in the a state, = 0.84 D (exp.) & 1.53 D (theory) and finally in the A state, = 0.15 D (exp.) & 0.539 D (theory). Thus, based on the environment in which CO exists in the condensed phase, the electronic states X, a and d which carry negative move energetically in the opposite direction (get energetically stabilised or destabilised) with respect to the electronic states a and A which carry positive . Keeping these aspects in mind we have analysed our emission spectra.
Using the spectroscopic constants (, and ) of a, a , d and e states of CO in Ar matrices obtained by Bahrdt et al. (1987), the energies of the vibrational states of the a , d and e electronic states were independently calculated with respect to the v =0 level of the a-state (Fig. 3). It was needed to shift the energies of the upper three electronic states (a by -41 cm-1, d by -183 cm-1 and e by -18 cm-1) given by Bahrdt et al. (1987) in order to have a good agreement with the energies of vibronic states measured in the present study (Table 1). Due to the uncertainties in the values extrapolated and that only the gas-phase and are available for the d and e states (Bahrdt et al. 1987), the above mentioned spectral shifts are acceptable. Possible second choice is also given for some of the bands in Table 1. It can clearly be seen that the choice of d(0) a(1 and 2) fits better with the bands at 10588 and 8897 cm-1 than the choice of a (6) a(2 and 3). The band at 7244 cm-1 can be assigned to either or both the choices given in Table 1. All the other possible assignments resulted in more than 100 cm-1 difference between the observed and calculated energies.
Table 1. Spectroscopic constants of the triplet states of CO in Argon matrices from Bahrdt et. al. (1987), the energies of the NIR triplet-triplet transitions calculated using these constants (calculated) and those measured in this work (measured). All values are given in cm-1.
In contrast to the gas-phase, where the radiative or nonradiative processes are controlled by electronic coupling (A) and vibrational () Franck-Condon factors, in condensed environment, for example in the interstellar grain particles (rare-gas matrices in the present case), an additional factor plays a crucial role. This factor is due to the coupling of phonons with the vibronic levels ( p o ). A detailed discussion can be found in the publication of Bahrdt & Schwentner (1988). It is sufficient to state here that the coupling constant V for nonradiative relaxation, which can be expressed as a product of the above three interactions: V = A p o , is insignificantly small among a , d and e states, but gets significant between a and a , d or e states. The vibrational levels of any particular electronic state having small values of the coupling constant V build so called bottlenecks where radiative relaxation competes with nonradiative relaxation. In general, it may be expected that the probability of emission to occur from a vibrational level of the a , d and e states increases with increasing gap between this vibrational level and the vibrational levels of the a-state that lies immediately bellow. Consequently, the bottlenecks can be identified as e(0), a (6), d(0) and a (2) from Fig. 3. Bahrdt (1987) has observed from three of these upper states, namely the e(0), a (6) and a (2) states, respectively, radiative transitions (in the VUV region) directly to the ground-state and our NIR spectra complement their observations.
© European Southern Observatory (ESO) 1998
Online publication: November 24, 1997