“…The sensitivity of CARS is low and the spatial resolution is limited, unless BOXCARS or folded BOXCARS arrangements are employed, but absolute calibration is possible since it is easy to detect H 2 ( v = 0). Vibrationally excited H 2 can be detected by LIF,161, 163 and absorption spectroscopy 93, 163, 164, 171. With these techniques, however, it is impossible to detect H 2 (v = 0), and hence to evaluate the H 2 ( v = 1)/H 2 ( v = 0) population ratio without using short‐wavelength radiation below the LiF cut‐off.…”