Molecular amplified spontaneous emission in the far-infrared region

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The 8sσ state of NO is weakly predissociative and no fluorescence has been reported at any spectral regions so far, whereas two major ASE systems were observed around 27 μm and 14 μm. Judging from the low quantum efficiency of the detector above 25 μm, the emission intensities of the 27 μm band is evaluated to be at least two orders of magnitude stronger than the 14 μm band.The 27 μm bandThe band system around 27 μm is assigned to the 8sσ ^2Σ^+(ν=0)→7pσ ^2Σ^+(ν=0) transition. The observed rotational structure accords with the R branch. The corresponding P branch is located above 28 μm. The transition down to the lower pσ Rydberg members such as 8sσ→6pσ at 6.6 μm is not detectable. The absence of the sσ→pσ transitions with Δn=2 has been also pointed out for the 6sσ state. The 8sσ→7pπ ^2Π system around 23.5 μm is undetectable. This may be caused by s 〜 d mixing in the 8sσ state, leading to destructive interference of the 8sσ→7pπ and 7dσ→7pπ transition dipoles. A similar intensity reducti … More on of the nsσ→(n-1)pπ ASE systems was found for n=4,5 and 6. Cascade ASE from 7pσ (ν=0)→7sσ (ν=0) around 19 μm was not observed.The 14 μm bandThe band system around 14 μm is assigned to the 8sσ(ν=0)→6f(ν=0) transition. This is the first observation of the nsσ→n'finter-Rydberg ASE which violates the Δl=±1 selection rule. The transition moment is derived from the mixing of dσ character with 8sσ. Our analyses revealed that the peaks belong to the P branch where the quantum number ι (projection of the orbital angular momentum l onto the rotational axis) in the 6f state is +3. This assignment is in good accordance with our previous investigation in which the 4f(v=0)→sσ E^2Σ^+ (ν=0) ASE channel was open only for ι=+3. When the 6f(ν=0,L=+3) levels are directly populated from A^2Σ^+(ν=0), the ASE spectra were dominated by the band around 8 μm assigned to the 6f→5dσ transition.The current observation implies that molecular ASE may be used as quasi-coherent pulsed light source in THz regions. As the higher (n【greater than or equal】9) Rydberg members are excited, the energy differences between the laser-prepared and the adjacent levels become smaller, potentially yielding longer ASE wavelengths. For instance, the energy difference between 10sσ ^2Σ^+(ν=0) and 9pσ ^2Σ^+(ν=0) states is〜165 cm^<-1>, corresponding to 60 μm (【approximately equal】 5.0 THz). Although continuous tuning is not feasible, robivational structures of molecules provide us with much wider choices of wavelength than atomic emissions. Less