過渡吸収測定とは、パルスレーザをサンプルに照射することによって励起状態分子やラジカル等の過渡種を瞬間的に発生させ、過渡種の吸収スペクトルやその時間変化を測定する手法です。この測定により、励起分子のダイナミクスといった励起状態についての情報や、ラジカルの検出による化合物の光分解の過程等の情報を得ることができるため、発光材料 / 発光デバイスの光物理、光化学過程の追跡が可能となります。
図1に過渡吸収測定系の模式図、図2に過渡吸収測定装置のシステム構成例を示します。測定には励起光(ポンプ光)と 検出光(プローブ光)という2種類の光および検出器を使用します。ポンプ光はサンプルを励起させるために使用し、プローブ光はポンプ光によって生じたサンプルの状態変化を検出するために使用します。一般的にポンプ光にはNd:YAGレーザ、プローブ光にはXeランプ等が使用されます。検出器にはPMT(光電子増倍管)やストリークカメラが使用されますが、浜松ホトニクスが提供する過渡吸収測定装置の検出器には分光器と組み合わせたストリークカメラが用いられます。
図1:過渡吸収測定系の模式図
図2:過渡吸収測定装置の構成例
図3は過渡吸収測定の原理を示したものです。
まず、サンプルにポンプ光(パルスレーザ)を照射すると、サンプル中の一部の分子がポンプ光を吸収し、励起状態となります。その後、分子が基底状態に戻るまでの間にプローブ光を測定することで、その励起状態分子の吸収スペクトルやその時間変化が得られます。
分子が励起状態になっている時、一般的にサンプルは基底状態の時とは違った吸収スペクトルを示します。励起されたサンプルは時間経過とともに中の分子が基底状態に戻ろうとしますが、全ての分子が基底状態に戻るまでの間に複数回吸収スペクトルを測定することで、時間経過に伴う吸収スペクトルの変化を測定することができます。
検出器にPMTを使用する場合、ポンプ光とプローブ光を繰り返し照射し測定のタイミングをずらしながら何度か測定をする必要がありますが、ストリークカメラの場合は時間分解能が高いため、ワンショットでの測定が可能になります。ワンショットでの撮影は測定時間が短縮できるだけでなく、光劣化をするサンプルの測定に向いています。
図3:過渡吸収測定の原理
データ提供:産業技術総合研究所 超臨界流体研究センター(仙台市)研究員 相澤 崇史 様
ピコ秒~ナノ秒の時間領域における過渡吸収スペクトルを測定します。シングルショットで多波長同時に時間分解測定することにより、不可逆的な現象をとらえることが可能です。
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