Holger Stiel

Two-photon excitation of alkyl-substituted magnesium phthalocyanine: radical formation via higher excited states

Abstract The photophysical properties of tetra-(tert-butyl)-phthalocyanato-magnesium (t 4 -PcMg) in solution and microheterogeneous systems (liposomes and micelles) were investigated. Radical cation formation occurs in chloroform during UV excitation in the presence of an electron acceptor (CBr 4 ). The same result is achieved by two-step absorption in the singlet manifold using pulsed excitation at λ exc =670 nm, which is of interest from the viewpoint of photon delivery through the therapeutic window of tissues. To obtain a deeper insight into the photophysics leading to radical cation formation via the higher excited singlet state, the transient spectra and singlet—singlet absorption cross-sections were determined. In addition to strong excited state absorption within the spectral range of the Q x -band, relatively large absorption cross-sections were also found in regions with low ground state absorption. The importance of these transitions for an effective two-colour excitation regime is discussed with regard to new start mechanisms for photodynamic laser tumour theraphy.

Spectroscopic properties of potential sensitizers for new photodynamic therapy start mechanisms via two-step excited electronic states

Three substituted tetraazaporphyrins, octa‐(4‐tert‐butylphenyl)‐tetrapyrazinoporphyrazine, tetra‐(4‐tert‐butyl)phthalocyanine and tetra‐(4‐tert‐butyi)phthalocyanatomagnesium (t4‐PcMg), were spectroscopically checked in solutions and liposomes with respect to suitability as potential sensitizers of a possible new start mechanism for photodynamic therapy (PDT) from a stepwise excited higher singlet state. This PDT start mechanism was recently proposed to overcome the problem of O2 (1δ2)‐caused cutaneous phototoxicity in PDT.

Quantitative comparison of excited state properties and intensity- dependent photosensitization by rose bengal.

A quantitative (multistep) excitation-deactivation model of rose bengal (RB) has been developed which includes energy and electron transfer to oxygen and the possibility of photoproduct formation via higher excited triplet-singlet states. The model is based on previous measurements of non-linear absorption (NLA) and emission of RB with picosecond pulses at 532 nm and on NLA measurements with nanosecond pulses. A coupled rate equation and photon transport equation approach for non-linear light-matter interaction is used. The resulting term scheme with all relevant excited state parameters confirms that (i) in the first excited state of RB, relevant absorption at 532 nm takes place only in the triplet, and (ii) the previously reported intensity dependence of RB-sensitized enzyme inhibition is well modelled by the intensity-dependent RB-T1 population and (as the main process) subsequent energy transfer to form singlet oxygen.

A high-density sub-micron liquid spray for laser driven radiation sources

A new and (simple) versatile high-density sub-micron liquid spray source has been developed and examined for use in laser–plasma interaction studies. Sub-micron sized water droplets were produced in a spray by hypersonic expansion of superheated water vapour through a nozzle into vacuum. Employing Mie scattering and transmission measurement of the spray, the size of individual droplets were determined to a diameter of d = (0.15 ± 0.01) µm. The droplet number density varied from (1.6 ± 0.5) × 1011 up to (7.5 ± 0.7) × 1010 droplets cm−3 depending on the position below the nozzle while the maximum mean atomic density was >1018 atoms cm−3. This water spray target system provides a six to eight times higher x-ray emission at photon energies below 10 keV compared to a single droplet (15 µm diameter) source where both were irradiated with ultrafast (35 fs) 20 TW Ti : Sa laser pulses.

Influence of laser intensity and pulse duration on the extreme ultraviolet yield from a water jet target laser plasma

Extreme ultraviolet (EUV) emission in the 11–15 nm wavelength range from a thin liquid water jet target under illumination with a high repetition rate, high average power laser (Nd-YLF) has been studied. To find the optimum conversion efficiency of laser light into EUV radiation, different laser parameters were applied. The laser intensity was varied between 1011 and 1015u200aW/cm2, and pulse duration in the range from 30 ps to 3 ns. A maximum conversion efficiency of 0.12% in 2.2% bandwidth and 4π steradian at 13 nm was achieved at a repetion rate of 250 kHz, and a strong dependence of the conversion efficiency on both laser intensity and pulse duration was found.

Intensity-dependent enzyme photosensitization using 532 nm nanosecond laser pulses.

Abstract— The intensity dependence of the rose bengal (RB)‐photosensitized inhibition of red blood cell acetylcholinesterase has been studied experimentally and the results compared to a quantitative excitation/deactivation model of RB photochemistry. Red blood cell membrane suspensions containing 5 μM RB were irradiated with 532 nm, 8 ns laser pulses with energies between 1 and 98.5 mJ. A constant dose (7 J) was delivered to all samples by varying the total number of pulses. At incident energies greater than ∼ 4.5 mJ/pulse, the efficiency for photosensitized enzyme inhibition decreased as the energy/pulse increased. The generation of RB triplet state was monitored as a function of laser energy and the triplet‐triplet absorption coefficient was determined to be 1.9 × 104M−1 cm−1 at 530 nm. The number of singlet oxygen molecules produced at each intensity was calculated from both the physico‐mathematical model and from laser flash photolysis results. The results indicated that the photosensitized inhibition of acetylcholinesterase was exclusively mediated by singlet oxygen, even at the highest laser intensities employed.

Ion acceleration with ultrafast lasers

Hot-electron confinement can build up fields capable of accelerating ions up to MeV energies when an ultrashort 35-fs laser pulse at ∼2×1018u2009W/cm2 interacts with a small spherical target. Singly charged ions with different masses have similar energies. A simple phenomenological model describes how ultrashort and less-energy-consumptive pulses drive ions to MeV energies. The energetic and spatial-emission characteristics of protons, deuterons and oxygen ions released from water and heavy-water droplets of ∼15u2009μm in size was determined for this interaction scenario.

Efficient extreme ultraviolet emission from xenon-cluster jet targets at high repetition rate laser illumination

We studied extreme ultraviolet (EUV) emission from Xe cluster jet targets irradiated with high repetitive ps and ns laser pulses in the intensity range between 1011 and 1015u200aW/cm2. It was found that at fixed intensity the conversion efficiency of the laser energy into the EUV emission is higher for ns pulses. In the intensity range used no saturation of the 13.4 nm signal could be reached. At ns pulse duration a conversion efficiency 0.26%@13.4 nm in 2π steradian and 2.2% bandwidth was reached. The laser energy deposition is discussed in relation to the plasma dynamics of the cluster target. We demonstrated that due to the high flow velocity of the cluster jet target an exposition with laser pulses up to 125 kHz repetition rate is possible without any degradation of the EUV emission efficiency. Both the high conversion efficiency and the high repetition rate make this Xe target attractive for an EUV source with high average EUV power.

Scaling-up a liquid water jet laser plasma source to high average power for Extreme Ultraviolet Lithography

In this article we describe a laser plasma source for Extreme Ultraviolet Lithography (EUVL) based on a liquid water jet target. Although jet targets are known for some time now, no attempts have been made to prove the functionality of the target under conditions similar to an EUVL production-line facility, that means illumination with high average power laser systems (in the multi-kW regime) at repetition rates in the kHz region. Such systems are currently under development. We used the MBI-burst laser to simulate these extreme illumination conditions. We examined the hydrodynamic stability of the target as a function of the laser repetition rate at different average laser powers (0.6kW and 5kW per burst). Additionally, the dependence of the conversion efficiency on pulse duration in the range from 30ps to 3ns was investigated. From our results one can conclude parameters for future design of driver lasers for EUVL systems.

One- and Two-Photon-Induced Photochemistry of Modified Palladium Porphyrazines Involving Molecular Oxygen

Abstract— Octaphenyltetraanthraporphyrazinato palladium undergoes a self‐sensitized photoreaction in the presence of oxygen to form a substituted palladium phthalocyanine with four endoperoxide bridges. This compound exhibits photophysical behavior similiar to palladium tetra‐tert‐butyl‐phthalocyanine. The phthalocyanine‐palladium complex with four endoperoxide bridges ejects molecular oxygen when excited by consecutive two‐photon absorption in the Q‐band region at 662 nm. This photocyclo‐reversion, which produces palladium porphyrazines bearing a diminished number of endoperoxide bridges, can occur up to four times per initial molecule. Irradiation of these photoproducts in the presence of oxygen produces substituted palladium phthalocyanine containing four endoperoxide groups.