Photoisomerisation quantum yield and non-linear cross-sections with femtosecond excitation of the photoactive yellow protein
Photoisomerisation quantum yield and non-linear cross-sections with femtosecond excitation of the photoactive yellow protein
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DOI：
10.1039/c2cp41718a
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发表时间：
2012-01-01
影响因子：
3.3
通讯作者：
van Thor, Jasper J.
中科院分区：
文献类型：
--
作者：
Lincoln, Craig N.;Fitzpatrick, Ann E.;van Thor, Jasper J.
在飞秒光激发下，光活性黄蛋白(PYP)的光异构化量子产率强烈地依赖于峰值功率和波长。利用系统功率滴定和二阶色散的加入，获得了140、300和600fs的脉冲宽度，并用暂态吸收光谱和Z扫描技术对400、450和490 nm处的单光子和多光子截面进行了估算。应用结合了光选择理论的靶模型，确定了第一激发态的受激发射和吸收截面、超快内转换量和与物质相关的动力学。在400、450和490 nm激发波长下，光异构化的最终量子产率分别为0.06、0.14-0.19和0.02，并且随着脉冲持续时间的增加而增加。与基态吸收的最大波长一致的450 nm处的瞬时吸收测量和Z扫描测量表明，光化学量子产率本质上受到超快内转换反应和受激发射截面的限制。在400 nm激发下，量子产率进一步受到竞争多光子激发进入385 nm激发态吸收的限制，这是先前提出的导致光电离的原因。当激发波长为490 nm时，光异构化量子产率主要受到比基态截面和多光子过程更大的受激发射截面的限制。除了光致电离外，还发现了多光子激发的第二个产物，其特征是在500 nm处有诱导吸收，弛豫时间常数为2ps。在功率密度高达138GW cm(-2)的情况下，测量结果并没有提供PYP的相干多光子吸收的迹象。在450 nm激发的饱和区，光异构化量子产率的极限为0.14-0.19，激发态吸收截面为6.1×10(-17)cm(2)，是基态截面1.68×10(-16)cm(2)的0.36倍。这从根本上限制了最大布居数和样品渗透率，例如在飞秒泵浦-探测实验中对PYP分子晶体的限制。
The quantum yield of photoisomerisation of the photoactive yellow protein (PYP) strongly depends on peak power and wavelength with femtosecond optical excitation. Using systematic power titrations and addition of second order dispersion resulting in 140, 300 and 600 fs pulse durations, the one and multi-photon cross-sections at 400, 450 and 490 nm have been assessed from transient absorption spectroscopy and additionally the Z-scan technique. Applying a target model that incorporates photoselection theory, estimates for the cross-sections for stimulated emission and absorption of the first excited state, the amount of ultrafast internal conversion and the underlying species associated dynamics have been determined. The final quantum yields for photoisomerisation were found to be 0.06, 0.14-0.19 and 0.02 for excitation wavelengths 400, 450 and 490 nm and found to increase with increasing pulse durations. Transient absorption measurements and Z-scan measurements at 450 nm, coinciding with the maximum wavelength of the ground state absorption, indicate that the photochemical quantum yield is intrinsically limited by an ultrafast internal conversion reaction as well as by stimulated emission cross-section. With excitation at 400 nm photoisomerisation quantum yield is further significantly limited by competing multi-photon excitation into excited state absorption at 385 nm previously proposed to result in photoionisation. With excitation at 490 nm the photoisomerisation quantum yield is predominantly limited further by the significantly higher stimulated emission cross-section compared to ground state cross-section as well as multi-photon processes. In addition to photoionisation, a second product of multi-photon excitation is identified and characterised by an induced absorption at 500 nm and a time constant of 2 ps for relaxation. With power densities up to 138 GW cm(-2) the measurements have not provided indication for coherent multi-photon absorption of PYP. In the saturation regime with 450 nm excitation, the limit for the photoisomerisation quantum yield was found to be 0.14-0.19 and the excited state absorption cross-section 6.1 x 10(-17) cm(2) or 0.36 times the ground state cross-section of 1.68 x 10(-16) cm(2) per molecule. This places a fundamental restriction on the maximum populations and sample penetration that may be achieved for instance in femtosecond pump-probe experiments with molecular crystals of PYP.