We performed UV spectroscopy for p-coumaric acid (pCA), ferulic acid (FA), and caffeic acid (CafA) under jet-cooled gas-phase conditions by using a laser-ablation source. These molecules showed the S₁(¹ππ*)–S₀ absorption in the 31 500–33 500 cm^–1 region. Both pCA and FA exhibited sharp vibronic bands, while CafA showed only a broad feature. The decay time profile of the ¹ππ* state was measured by picosecond pump–probe spectroscopy, and the transient state produced through the nonradiative decay (NRD) from ¹ππ* and its time profile were measured by nanosecond UV–deep UV pump–probe spectroscopy. The transient state was observed for pCA and FA and assigned to the T₁ state, and we concluded that the NRD process of ¹ππ* is S₁(¹ππ*) → ¹nπ* → T₁(³ππ*), similar to those of methyl cinnamate and para-substituted cinnamates such as p-hydroxy and p-methoxy methyl cinnamate. On the other hand, the transient T₁ state was not detected in CafA, and its NRD route is suggested to be S₁(¹ππ*) → ¹πσ* → H atom elimination, similar to those of phenol and catechol. The effect of a hydrogen bond on the electronic state and NRD process was investigated, and it was found that the H-bonding lowers the ¹ππ* energy and suppresses the NRD process for all the species.

中文翻译：

---

激光消融技术研究冷气相肉桂酸衍生物的电子态和非辐射衰减。

我们使用激光烧蚀源在喷射冷却的气相条件下对对香豆酸（p CA），阿魏酸（FA）和咖啡酸（CafA）进行了紫外光谱分析。这些分子表现出在S ₁（¹ ππ*） - S- ₀吸收在31 500-33500厘米^-1区域。两个p CA和FA表现出尖锐的振动带，而CAFA仅表现出广泛的功能。所述的衰减时间简档^1个ππ*状态通过皮秒泵浦-探测光谱测量，并且通过从所述非辐射衰变（NRD）中产生的过渡状态¹ππ*及其时间分布是通过纳秒级紫外-深紫外泵浦-探针光谱法测量的。观察到过渡状态为p CA和FA和分配到T ₁的状态，我们的结论是，NRD过程¹ ππ*是S ₁（¹ ππ*）→ ¹ Nπ*→T的₁（³ ππ*），类似于肉桂酸甲酯和对位肉桂酸酯，例如对羟基和对甲氧基甲基肉桂酸酯。在另一方面，所述瞬态Ť ₁在咖啡厅中未检测到状态，并且它的NRD路线建议为S ₁（¹ ππ*）→^1个πσ*→H原子消除，类似于那些酚和儿茶酚的。上的电子状态和NRD过程氢键的影响进行了研究，并且发现，该氢键降低了^1个ππ*能量和抑制NRD过程对于所有的物种。