产生的条件： 条件一:辐射光子的能量应与振动跃迁所需能量相等。 根据量子力学原理，分子振动能量Ev是量子化的,即Ev= (V+1/2) hv “v-振动频率” “V-振动量子数，值取0，1, 2，...” 只有当Ea=△Ev时，才可能发生振转跃迁。 相邻两个能级间的能量差ΔEv= hv — 条件二:产生偶极矩变化的振动 μ=qXd（μ为矢量，正电中心指向负电中心） 对称分子: μ=0,不产生红外吸收，如: N2、 O2、Cl2 等。 非对称分子:μ≠0, 红外“活性”振动，产生红外吸收。
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I had no idea how difficult Sondheim's music would be. All through the rehearsals, I kept flubbing. There were so many tempo changes. I could never get through the opening number without any mistakes. One day, I went up to Hal Prince and offered to leave the show. He laughed it off. He said, 'Don't be silly. That's why we have tryouts.'