Single-bubble sonoluminescence (SBSL) from a hydrogen bubble is studied theoretically based on a quasiadiabatic compression model of a bubble collapse. It is clarified that the maximum temperature in a hydrogen bubble in 20 deg C water under conditions of SBSL is always about 6000 K due to the effect of chemical reactions inside the bubble. It is suggested that the light emission at such temperature is by the transition from the lowest stable triplet state of the H_2 molecule to the repulsive state resulting from two normal atoms (H_2~* -> 2H + hv). It is shown that the number of hydrogen molecules inside the bubble remains almost constant in spite of the high temperature and pressure inside the bubble at the collapse. It is also shown that the addition of argon to a hydrogen bubble results in the higher maximum temperature inside the bubble.
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