Abstract:We perform multi-photon direct laser writing as a function of laser repetition rate over many orders of magnitude and otherwise unchanged experimental conditions. These new data serve as basis for investigating the influence of different proposed mechanisms involved in the photopolymerization: two-photon absorption, photoionization, avalanche ionization and heat accumulation. We find different non-linearities for high and low repetition rates consistent with different initiation processes being involved. The scaling of the resulting linewidths, however, is neither expected nor found to depend on repetition rate or non-linearity. References and links1. H. B. Sun, S. Matsuo, and H. Misawa, "Three-dimensional photonic crystal structures achieved with two-photonabsorption photopolymerization of resin," Appl. Phys. Lett. 74, 786-788 (1999). 2. S. Kawata, H. B. Sun, T. Tanaka, and K. Takada, "Finer features for functional microdevices," Nature 412, 697-698 (2001). 3. M. Straub and M. Gu, "Near-infrared photonic crystals with higher-order bandgaps generated by two-photon photopolymerization," Opt. Lett. 27, 1824Lett. 27, -1826Lett. 27, (2002

Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n'arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. Questions? Contact the NRC Publications Archive team atPublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information. NRC Publications Archive Archives des publications du CNRCThis publication could be one of several versions: author's original, accepted manuscript or the publisher's version. / La version de cette publication peut être l'une des suivantes : la version prépublication de l'auteur, la version acceptée du manuscrit ou la version de l'éditeur. For the publisher's version, please access the DOI link below./ Pour consulter la version de l'éditeur, utilisez le lien DOI ci-dessous.http://doi.org/10.1021/acs.jpca.5b04900Access and use of this website and the material on it are subject to the Terms and Conditions set forth at Ultrafast dynamics of o-nitrophenol: An experimental and theoretical study Ernst, Hanna A.; Wolf, Thomas J. A.; Schalk, Oliver; González-García, Nuria; Boguslavskiy, Andrey E.; Stolow, Albert; Olzmann, Matthias; Unterreiner, Andreas-Neil http://nparc.cisti-icist.nrc-cnrc.gc.ca/fra/droits L'accès à ce site Web et l'utilisation de son contenu sont assujettis aux conditions présentées dans le site LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D'UTILISER CE SITE WEB. NRC Publications Record / Notice d'Archives des publications de CNRC:http://nparc.cisti-icist.nrc-cnrc.gc.ca/eng/view/object/?id=ce83ef5b-a095-4e1f-85dd-e4e329f4a190 http://nparc.cisti-icist.nrc-cnrc.gc.ca/fra/voir/objet/?id=ce83ef5b-a095-4e1f-85dd-e4e329f4a190 ABSTRACT: The photolysis of o-nitrophenol (o-NP), a typical push−pull molecule, is of current interest in atmospheric chemistry as a possible source of nitrous acid (HONO). To characterize the largely unknown photolysis mechanism, the dynamics of the lowest lying excited singlet state (S 1 )o fo-NP was investigated by means of femtosecond transient absorption spectroscopy in solution, time-resolved photoelectron spectroscopy (TRPES) in the gas phase and quantum chemical calculations. Evidence of the unstable aci-nitro isomer is provided both in the liquid and in the gas phase. Our results indicate that the S 1 state displays strong charge transfer character, which triggers excited state proton transfer from the OH to the NO 2 group as evidenced by a temporal shift of 20 fs of the onset of the photoelectron spectrum. The proton transfer itself is found to be coupled to an out-ofplane rotation of the newly formed HONO group, finally leading to a conical intersection between S 1 and the ground state S 0 . In solution, return to S 0 within 0.2−0.3 ps was monitored by stimulated emission. As a competitive relaxation channel, ultrafast intersystem cro...

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The excited states and dynamics of the three triplet radical photoinitiators benzoin (2-hydroxy-1,2-diphenylethanone, Bz), 2,4,6-trimethylbenzoin (2-hydroxy-1-mesityl-2-phenylethanone, TMB), and mesitil (1,2-bis(2,4,6-trimethylphenyl)-1,2-ethanedione, Me)employed in our previous studies for quantifying net initiation efficiencies in pulsed laser polymerization with methacrylate monomers [Voll, D.; Junkers, T.; Barner-Kowollik, C. Macromolecules 2011, 44, 2542–2551]are investigated via both femtosecond transient absorption (TA) spectroscopy and density functional theory (DFT) methods to elucidate the underlying mechanisms causing different initiation efficiencies when excited at 351 nm. Bz and TMB are found to have very similar properties in the calculated excited states as well as in the experimentally observed dynamics. After excitation into the first excited singlet state (S1) Bz and TMB undergo rapid intersystem crossing (ISC). The ISC can compete with ultrafast internal conversion (IC) processes because an excited triplet state (T n ) of nearly the same energy is present in both cases. ISC is therefore the dominating depopulation channel of S1, and subsequent α-cleavage to produce radicals takes place on the picosecond time scale. In contrast, Me is excited into the second excited singlet state (S2). In this case no isoenergetic triplet state is available, which inhibits ISC from competing with ultrafast deactivation processes. ISC is therefore assigned to be a minor deactivation channel in Me. Employing these findings, quantitative photoinitiation efficiency relations of Bz, TMB, and Me obtained by pulsed laser polymerization can be directly correlated with the relative TA intensities found in the femtosecond experiments. The ISC efficiency is thus a critical parameter for evaluating the overall photoinitiation efficiency and demonstrates that the employment of the herein presented method represents a powerful tool for attaining a quantitative picture on the suitability of a photoinitiator.

We report on femtosecond pump-probe experiments on two different photoinitiators in solution. These two molecules have recently appeared as attractive candidates for far-field optical lithography based on stimulated-emission-depletion (STED) inspired approaches aiming at beating Abbe's diffraction limit. For the case of 7-diethylamino-3-thenoylcoumarin (DETC), we find that stimulated emission clearly dominates over excited-state absorption, whereas the opposite holds true for the case of isopropylthioxanthone. We argue that it is desirable that stimulated emission dominates over excited-state absorption as depletion mechanism in STED photoresists. Thus, DETC is an attractive corresponding photoinitiator.

The ultrafast relaxation dynamics of the well-known solvated electron in liquid ammonia solutions are investigated with femtosecond near-infrared pump-probe absorption spectroscopy. Immediately after photoexcitation, the dynamic absorption spectrum of the electron is substantially red-shifted with respect to its stationary spectrum. A subsequent dynamic blue shift of the pump-probe spectrum occurs on a timescale of 150 fs. The data are understood in terms of ground-state "cooling" and can be quantitatively simulated by an intuitive temperature-jump model employing a dynamically evolving Kubo line shape for the electronic resonance. A simple estimate implies that, on average, the electron in the liquid is coordinated to six nearest-neighbor ammonia molecules. An equivalent analysis of the data based on a bubble-formation/cavity-contraction mechanism is briefly outlined.

The fundamental influence of the structure and substitution of radical photoinitiators was investigated via a trifold combination of pulsed-laser polymerization with subsequent electrospray-ionization mass spectrometry (PLP-ESI-MS), femtosecond transient absorption (fs-TA) spectroscopy, and quantum chemistry. For the first time, a library of benzoin-derived photoinitiators with varied substitution patterns was synthesized. In the PLP-ESI-MS study, different photoinitiators were compared pairwise in so-called cocktail experiments, enabling the direct comparison of their initiation efficiency. In the fs-TA experiments, the transient response was observed after UV excitation in the visible spectral region, allowing for a description of excited state dynamics, which was analyzed with the aid of TD-DFT calculations. Ab initio calculations were undertaken to determine the reactivity of the radical fragments generated from these photoinitiators and to quantify the influence of various substituents on the rate of addition to monomer. In summary, the influence of the substituent on the initiation efficiency, intersystem crossing (ISC) behavior, excited state dynamics, and the extinction coefficients were analyzed. Hence, relaxation pathways and reaction mechanisms were optimized to explain disparate initiation efficiencies of a wide range of newly designed photoinitiators with varying substitution patterns. The strongly divergent absorptivities of the different photoinitiators and their corresponding initiation efficiencies underline that the absorptivity of a molecule is by no means an unequivocal measure for its reactivity when excited at a specific wavelength. In fact, the most efficient initiators are governed by one nπ* singlet state with a very low extinction coefficient at the excitation wavelength and one or two triplet states with nπ* character.

Femtosecond time-resolved pump-probe photoelectron spectroscopy was used to study elementary relaxation processes occurring in isolated phthalocyanine-tetrasulfonate tetra-anions ([MPc(SO3)4]4-, M=Cu,Ni, and "free-base" [H2Pc(SO3)4]4-) following Q band excitation by one-photon absorption at 775 nm. Whereas the Cu and Ni systems decay rapidly by means of internal conversion without electron loss, the free-base phthalocyanine primarily undergoes excited state tunneling electron emission. This reflects less efficient coupling to lower lying states within the corresponding spin manifold. Results are interpreted in terms of (time-dependent) density functional theory calculations of ground and electronically excited states and kinetically modeled to yield the associated rates.