Linear and nonlinear spectroscopic techniques applied to study of transient molecular species

Carbon chain radicals and ions have long been known to exist in the interstellar medium, and considered as potential carriers for diffuse interstellar bands. Measurements on these molecules are difficult because they are short-lived species generated in small number densities. Discharge and laser vaporization sources coupled to a supersonic jet expansion are, so far, the most effective techniques used to generate sufficiently large densities suitable for spectroscopic studies of these transients. The present work describes the combination of these molecular sources with high resolution cavity ring down (CRDS) and four wave mixing (FWM) spectroscopic techniques applied to detection of radicals and ions of astrophysical significance. CRDS offers high sensitivity because of the large absorption path lengths achieved inside the cavity and its immunity towards shot-to-shot laser fluctuations. Four-wave mixing, on the other hand, offers selectivity of the species studied by the application of very short discharge pulse lengths, in the nanosecond time scale. As a consequence, the molecules are separated out in the plasma discharge based on their masses.
The 3Π-3Π electronic transition of C6H+ has been measured using CRDS; this is the first gas phase detection of the cation. Partially resolved P lines and observation of band heads permitted a rotational contour fit. Spectroscopic constants in the ground and excited-state were determined. Broadening of the spectral lines indicates the excited-state lifetime to be around 100 ps. The potential of degenerate and two-color FWM applied to selectivity of transient species has been studied extensively in this work using various molecular systems like C3/C4H, C3/HC2S and C2/HC4H+, only by varying the timings between the experimental components (laser/valve/discharge) while applying extremely short (<1µs) discharge pulses. The two color variant is exceptionally powerful in disentangling overlapping features even within the same spectroscopic system. This is demonstrated in the case of HC4H+ where P lines of the Ω=3/2 spin orbit component are effectively separated out from the overlapping Ω=1/2 component in the A←X electronic transition. The first ever detections of ions (HC4H+, C2-) by FWM are also reported. The results suggest convincingly that nonlinear four-wave mixing spectroscopy is applicable to study numerous neutral, cationic and anionic radicals that are produced in plasma environments in low particle densities by applying a discharged free-jet expansion.
Both CRDS and FWM have been employed as tools for spectroscopic investigation of non-adiabatic effects in linear polyatomic molecules. The excitation of the ν3 (C-C stretch) and the 2ν7 (C≡C-C bend) levels in the A2Π electronic state of diacetylene cations results in Renner-Teller (R-T) and Fermi interactions. The ν3 and 2ν7 vibronic bands in the A←X transition of HC4H+ have been measured with rotational resolution using CRDS in a supersonic slit jet discharge. A vibronic analysis has been carried out taking into consideration the R-T, spin-orbit, and Fermi resonance interactions between the ν3 and ν7 modes. The spectroscopic constants for the excited electronic state are compared with the ground state. The double resonance four wave mixing approach was used to unambiguously identify the vibronic R-T manifold in the A2Π state up to 700 cm-1 above v=0 of C4H by pumping on the origin B←X electronic transition. On the basis of the experimental linelist, several of the energy levels are assigned to vibrations in the electronic X2Σ+ ground state. An assignment of the levels was carried out by R-T analysis, leading to a relatively large ε6 in the ground state for the second lowest bending mode as previously found in the upper state. This study results in the detection of levels located below the A2Π state because of high R-T interaction.
CRDS has also been employed to detect broad absorption features of the B←X transition of H2CCC (l-C3H2). The observations provide evidence that the broad, diffuse interstellar bands (DIBs) at 4881 and 5450 Å are caused by the B←X transition of H2CCC (l-C3H2). The large widths of the bands are due to the short lifetime of the B 1B1 electronic state. The bands in the gas phase show exact matches to the profiles and wavelengths of the two broad DIBs. This makes l-C3H2 a carrier of the DIBs, which have remained a long standing mystery in astronomy.
The present work provides an insight to understanding not only the fundamental spectroscopic properties of these transient species but also their astrophysical significance. Moreover, it also demonstrates the highly sensitive and selective capabilities of the employed experimental techniques, which could be of use in other fields like combustion, trace gas analysis, to name a few.