Near-infrared diode laser spectroscopy of FeC in the 0.8-μm region:: A simultaneous analysis of the X3Δi and [3.8]1Δ states

The near-infrared absorption spectra of the FeC radical have been measured by diode laser spectroscopy in the regions from 11 810 to 13 200 cm(-1). Rotationally resolved 28 new vibronic bands of the (FeC)-Fe-56-C-12 major isotopic species were observed as well as two known bands. Five of them were assigned as the v ' = v " = 0-4 sequence bands in the [13.1](3)Phi (4) <-- X-3 Delta (3) system, six are the V ' = V " = 0-5 sequences in the [13.18]Omega = 3 <-- X-3 Delta (2) system, and five were assigned as the V ' = V " = 0-4 sequences in the [13.5]Omega = 2 <-- X-3 Delta (1) system. The X(3)Delta (1) state of FeC is reported for the first time but we found that the rotational constant for this state of (FeC)-Fe-56 is very close to that for the X-3 Delta (3) state of (FeC)-Fe-54. Transitions of the (FeC)-Fe-54 minor isotopic species for the two known bands were thus reinvestigated. For the unclassified I I new bands, the lower states were assigned as the v = 0-3 states in X(3)Delta (i). The energy difference between the Omega = 2 and 3 sublevels in the X-3 Delta (i) state could be determined to be 329.8059 +/- 0.0005 cm(-1) as a results of the analysis of the two of new bands, [12.32] <-- X-3 Delta (3)v = 0 and [12.32] <-- X-3 Delta (2)v = 0 observed near 12317 cm(-1) and 11987 cm(-1), respectively. Stimulated emission pumping spectra were also recorded near 17600 cm(-1) to examine the low-lying electronic state which had been observed 3460 cm(-1) above the X-3 Delta (2) state (K. Aiuchi and K. Tsuji and K. Shibuya, Chem. Phys. Lett. 309, 229-233 (1999)), and it was assigned as the [3.8](1) A state arising from the same configuration as that of X(3)Delta (i) (...3 pi (4)1 delta (3)9 sigma (1)). In addition, the v = 0-0 and 1-1 bands of the [16.2]Omega = 3 <-- [3.8](1) Delta system were observed by near-infrared diode laser spectroscopy. The lower state combination differences were calculated for the possible sets of the energy levels in the X'Ai and [3.8]'A states with v = 0-5 by using the present result and the other spectroscopic data reported. A simultaneous analysis was carried out by using the combination differences to determine the molecular constants for the X-3 Delta (i) and [3.8](1) Delta states as well as the off-diagonal spin-orbit interaction terrns. The [13.18]Omega = 3 state and the [13.5]Omega = 2 state were concluded to be other two spin sublevels for the [13.1](3)Phi (4) state and the [16.2]Omega = 3 state was assigned as an isoconfigurational (1)Phi state. (C) 2001 Academic Press.