We present the results of highly excited H2 observations in photodissociation regions (PDRs); Orion Bright Bar, NGC7027 (Usuda et al. 1998), NGC2023, and NGC7023 (Takami et al. 1998).
We obtained the continuum-subtracted maps of the H2 v=-1S(1) and 4-2S(1) emission in J-band, using the near-IR Fabry-Perot imager (MUSE) (Takami et al. 1998), and made K-band slit spectroscopic observations with OASIS and UKIRT CGS4.<br>

The near-IR H2 emission is one of important lines to investigate the physical conditions of starforming regions and starburst galaxies.
In this sense, it is important issues in the study of the H2emission lines to make clear their excitation mechanism: collisional or UV radiative.
In general, lower vibrational H2 lines (e.g.,v=1-0S(1), 2-1S(1)) have been used to distinguish the excitation mechanisms.
However, recent researches have made clear that the H2 2-1S(1)/1-0S(1) intensity ratio cannot determine the two cases: denser gas in PDRs contribute to decrease the ratio or shocked regions are mixed with lower-density PDRs, except for the case that each H2 emitting region is spatially resolved (e.g., Burton et al. 1990, Usuda et al. 1996).
Thus we made the observations of higher vibrational transitions (v>=3), whose energy levels (T>=20000 K) are quite higher than the temperature of shocked gas (T=1000-2000 K), in order to confirm the excitation mechanism.<br>
 
We found the following results:<br>

(1) In the reflection nebulae NGC2023 and NGC7023, which are typical PDRs with weak UV radiation (G0=10^3-10^4), the intensity ratio 4-2S(1)/3-1S(1)=1 is consistent with a theoretical model of purely UV fluorescent emission (Black & van Dishoeck 1987, Takami et al. 1998).
This ratio is larger than those in shocked gas (e.g., 0.1-0.4).
s the values of the ratio are quite different, it is useful method to determine the H2 excitation mechanism.)<br>
(2) In Orion Bright Bar, a typical PDR whose UV radiation is strong (G0=10^5), the high vibration al lines were not detected in J-band.
In comparing the intensity of 1-0S(1) with 1-0O(5) (Sloan et al. 1997), the extinction is estimated as A_V=10, hence the non-detection in J-band can be explained by the effect of extinction (Usuda et al. 1998).<br>
(3) The K-band high-vib. lines were detected in Orion Bright Bar and NGC7027, and the intensity ratios (e.g., 3-2S(3)/1-0S(1) = 0.07) are consistent with the theoretical result (e.g., Burton et al. 1990).
In NGC7027, the 3-2S(2) intensity exceeds the values derived from standard models for UV fluorescence significantly, which may be caused by another excitation process such as a formation pumping mechanism.