Determination of the contributions of two simultaneous absorption orders using 2-beam action spectroscopy

Elucidating complex triplet-state dynamics in the model system isopropylthioxanthone

We introduce techniques for probing the dynamics of triplet states. We employ these tools, along with conventional techniques, to develop a detailed understanding of a complex chemical system: a negative-tone, radical photoresist for multiphoton absorption polymerization in which isopropylthioxanthone (ITX) is the photoinitiator. This work reveals that the same color of light used for the 2-photon excitation of ITX, leading to population of the triplet manifold through intersystem crossing, also depletes this triplet population via linear absorption followed by reverse intersystem crossing (RISC). Using spectroscopic tools and kinetic modeling, we identify the reactive triplet state and a non-reactive reservoir triplet state. We present compelling evidence that the deactivation channel involves RISC from an excited triplet state to a highly vibrationally excited level of the electronic ground state. The work described here offers the enticing possibility of understanding, and ultimately controlling, the photochemistry and photophysics of a broad range of triplet processes.

Extracting Information on Linear and Nonlinear Absorption from Two-Beam Action Spectroscopy Data

Two-beam action (2-BA) spectroscopies are a recently developed class of techniques for determining the order(s) of absorption (one-photon, two-photon, etc.) that contribute to an observable signal. When only a single order of absorption is present, 2-BA spectroscopies allow for the determination of that order from data obtained at a single value of the observable. It has been shown previously that when two orders of absorption are present, they can be determined unambiguously from measurements made at several values of the observable. However, this latter approach cannot be used for single-valued observables, such as a polymerization threshold. Here we develop a theoretical comparison between conventional methods that determine the order(s) of absorption using logarithmic plots and 2-BA-based techniques. We also explore how 2-BA plots arising from two orders of absorption deviate from a plot with a single, noninteger exponent. We demonstrate that these deviations can usually be used to identify the two orders of absorption and their relative contributions to the signal on the basis of measurements made at a single value of the observable.

Probing Multiphoton Photophysics Using Two-Beam Action Spectroscopy

Multiphoton absorption (MPA) is an enabling technology for many applications. However, due to the low probability of MPA processes, their accurate characterization remains a challenge. Here we introduce a new technique, two-beam constant emission intensity (2-BCEIn) spectroscopy, that offers substantial advantages over other existing methods that use the generation of optical emission for the characterization of absorptive nonlinearities. We use 2-BCEIn to study nonlinear absorption in solutions of crystal violet lactone (CVL) over a range of excitation wavelengths in which the dominant nonlinear absorption process transitions from two-photon absorption (750 nm) to three-photon absorption (830 nm). At an excitation wavelength of 800 nm, both two-photon absorption and three-photon absorption contribute substantially to the nonlinear fluorescence excitation (NFE) signal, although the dynamic range of the NFE data is not sufficient to quantify the contributions of each process. 2-BCEIn spectroscopy enables the direct measurement of the local exponent at each emission intensity. 2-BCEIn measurements made at several different emission intensities demonstrate unambiguously that the nonlinear excitation of CVL at 800 nm cannot be described solely as the sum of a two-photon process and a three-photon process. A kinetic model that includes intrapulse excited-state absorption reproduces the features of the 2-BCEIn measurements and enables the determination of the ratio of the three-photon absorption cross section to the two-photon absorption cross section. Such information cannot easily be extracted from conventional NFE measurements. These results demonstrate the power and versatility of two-beam action spectroscopies for elucidating the complex photophysics of multiphoton absorption processes.