4.3 UV–VUV Cross Correlation
4.3.1 UV–VUV Photoionization of Noble Gases
Earlier, the instrument response function for an experimental scheme has been determined utilizing the non-resonant multi-photon ionization of the noble gases xenon and krypton. Multi-photon ionization is of course also possible by a combination of third and fifth harmonic radiation.
Both, Kr and Xe exhibit a rich spectrum of Rydberg excitations, which might be accessed by a combination of third and fifth harmonic. Figure4.19gives an overview of states, which can be resonantly excited by one, two or three photon absorption. As in the previous section absorption
J = 1 J = 0, 2 J = 1,3 8
9 10 11 12 13 14
Potential energy / eV
J = 1 J = 0, 2 J = 1,3 8
9 10 11 12 13 14
Potential energy / eV
a) Kr energy levels b) Xe energy levels
2 UV
UV + VUV 3 UV
FIGURE4.19–Electronic states of krypton (a) and xenon (b) grouped by selection rules. Excitation schemes with combinations of UV and VUV are indicated.
- 5 0 0 - 4 0 0 - 3 0 0 - 2 0 0 - 1 0 0 0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 0 . 0
0 . 2 0 . 4 0 . 6 0 . 8 1 . 0 1 . 2
τ- 1 = 5 4 f s
τ1 = 1 1 f s τ2 = 7 2 f s τ3 > 1 0 p s
Ion signal / arb. units
U V - V U V d e l a y / f s K r+
FIGURE4.20–Delay-dependent ion yield for Kr+in a UV–VUV pump-probe scheme.
of three UV photons in Kr (Fig.4.19a) excites Rydberg states close to the first ionization potential.
Also a combination of both pulses might excite states by two photon absorption.
Rydberg excitation typically exhibits life times, which are on the order of picoseconds. In a two-color scheme, where pump and probe are not equivalent, one pulse, for example the third harmonic, may prepare a Rydberg excitation, which can be interrogated by single-photon ionization utilizing photons from the fifth harmonic pulse. Due to the long life time, this leads to a slowly decreasing delay-dependent ionization yield and an asymmetry, since interchanging third and fifth harmonic at negative delays does not prepare the same states.
This is exactly the case in a UV–VUV pump-probe experiment with krypton. Figure4.20shows the delay-dependent ion yield for Kr+. In all presented UV–VUV pump-probe data sets the convention is, that for positive delays the UV pulse arrives first and thus is termed the pump pulse, while the VUV pulse probes the system. At negative delays this is vice-versa: The VUV pulse arrives early and pumps the system, which is then interrogated by a UV probe pulse.
Compared to the previously presented experiments, the delay-dependent ion yield is more complex and cannot be modeled by a Gaussian function or the convolution between instrument response function and a single exponential decay function. To analyze the data a fit procedure has been implemented in the Matlab programming environment, to optimize a model of the type
S(t) = ΓIRF(t)⊗
n
X
i
Aiexp
−t τi
| {z }
fort≥0
+
m
X
j
Ajexp t
τj
| {z }
fort≤0
. (4.10)
ΓIRF(t)is the instrument response function, expressed as a Gaussian function, which is convo-luted with a sum of exponential decay function with a weightAi,jand a decay constant ofτi,j
either for a UV–VUV pump-probe scheme (t≥0) or the VUV–UV pump-probe scheme (t≤0).
Heretdefines the delay between UV and VUV pulse to avoid confusion with the decay constant τ. The FWHM ofΓIRF(t)is calculated from Eqn.4.8, using a pulse duration of 21 fs for the third harmonic and 16 fs for the fifth harmonic in a 2UV + 1VUV photon scheme.
The dominant contribution to the delay-dependent ion yield is a slowly decaying exponential functionτ3 >10ps, which appears, when the UV pulse arrives early and can be attributed the
4.3. UV–VUV CROSS CORRELATION
- 5 0 0 - 4 0 0 - 3 0 0 - 2 0 0 - 1 0 0 0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0
0 . 0 0 . 2 0 . 4 0 . 6 0 . 8 1 . 0 1 . 2
X e + τ- 1 = 1 1 f s τ- 2 = 8 4 0 f s
Ion signal / arb. units
U V - V U V d e l a y / f s
τ1 = 1 9 f s τ2 = 2 2 7 f s
FIGURE4.21–Delay-dependent ion yield for Xe+in a UV–VUV pump-probe scheme.
population of high-lying Rydberg states by three photon absorption (Fig4.19a). Furthermore, faster decay components are observed:τ1= 11fs is likely to due residual IR radiation probing the Rydberg excitation prepared by the UV pulse. Although the transmission of IR is below 0.1%, ionization from these Rydberg state is more likely for longer wavelengths. The origin ofτ2= 72fs andτ−1= 54fs is still an open question that exceeds the scope of this measurement. For example, the relevance of auto-ionization states or other short-lived state should be investigated in the future.
The original goal, an experimental determination of the instrument response function, could not be fulfilled with this measurement, since the observed delay-dependent signal is a convolution of the instrument response and its intrinsic dynamics.
The same is true for a measurement, where xenon is used as a target (Fig.4.21). Again multiple delay components are observed, both for negative and positive delays.
Although no direct resonances are accesible in Xe (see Fig.4.19b), an AC stark shift of less than 0.2 eV is sufficient to resonantly excite high lying Rydberg states by simultaneous absorption of a UV and VUV photon. Futhermore, this shift increases the binding energy of the electron and might close the UV+VUV two-photon ionization pathway.
Again, small time constants (τ1= 19 s andτ−1= 11fs) are observed, which indicate an influence of residual IR radiation. The slowly decaying componentτ−2= 840fs is attributed to population of AC-stark shifted Rydberg state by simultaneous absorption of a UV and VUV photon. Since this is happening also at longer delays, a fraction of UV radiation must be contained in the fifth harmonic pulse. When the UV pulse arrives early another time constantτ2= 227fs is observed.
This time constant is short for a Rydberg state life time, but might be due to population of auto-ionization states located above the first auto-ionization potential, stemming from spin-orbit splitting (see Fig.4.19).
Additional measurements have been performed in argon, which are shown in the appendixA.2.
For the same reasons as in krypton and xenon a long-lived exponential component is observed in the delay-dependent ion yield. Consequently, it can be concluded that multi-photon ionization of noble gases with a combination of 268-nm and 161-nm radiation is not an ideal non-linear process to retrieve the cross correlation of these pulses. Excitation of Rydberg and auto-ionization states induces delay-dependent features, which hinder the retrieval of the instrument response function.
268 nm pulse
100 fs
(26.1±1.7) fs FWHMIRF=
a) b)
c)
FIGURE4.22–Excitation scheme and delay-dependent ion yield of O2. (a) Excitation pathway I UV + VUV photon, Excitation path II VUV + 2 UV photons. (b) O2+spatial distribution in the common UV + VUV focal region. (c) Delay-dependent O2+yield with subtracted offset.