Stokes Components of Raman-Induced Singlet Oxygen

1. Introduction

Singlet oxygen (¹O₂) is a reactive oxygen species (ROS) widely used in photochemistry, photobiology, and photomedicine [1,2,3,4,5,6]. It corresponds to the lowest excited electronic level of the oxygen molecule. ¹O₂ excitation by direct photon absorption is not effective since the dipolar transition from ground to the ¹O₂ state is heavily forbidden by quantum mechanics selection rules. Photosensitizing is the commonly accepted ¹O₂ excitation method [7,8]. A large molecule that can absorb visible photons accumulates part of the absorbed energy in a metastable triplet state; from there, the energy can be transferred to oxygen molecules for ¹O₂ excitation. We have reported a new Raman-based, photosensitizer-free method for ¹O₂ photoexcitation [9]. A pumping photon generates a Stokes photon of lower energy, inducing the transition from the ground state to the ¹O₂ level. The energy difference between the two photons corresponds to the ¹O₂ energy level. We demonstrated the Raman character of the observed ¹O₂ Stokes signal, measured the ¹O₂ phosphorescence at 1270 nm, and determined the efficiency of the ¹O₂ Raman photoexcitation [10,11,12]. In stimulated Raman experiments on liquids such as water, the Stokes signals exhibit a characteristic diffraction pattern with a central spot and a system of rings [10,13]. Stokes signals corresponding to the solvent’s vibration modes overlap with the ¹O₂ Stokes in both parts of the diffraction pattern, making detection difficult. However, Stokes photons from water usually have lower energy than those from ¹O₂, making it possible to separate the contributions using long-pass spectroscopic filters. Furthermore, the oxygen Stokes peaks are of electronic origin and exhibit a wide spectral width of tens of nanometers. Meanwhile, the solvent Stokes peaks have a spectral width one order of magnitude smaller [9]. This significant difference helps identify peaks and separate them from one another.

The spectral response of molecular oxygen’s lowest electronic levels has been studied in detail for over a century. Analyzing the atmospheric absorption of solar radiation, Herzberg reported peaks at 1268 and 1067 nm [14,15]. The peaks were assigned to the absorption bands corresponding to transitions from the ground state to the two lowest vibrational modes of the ¹O₂ state. The results were confirmed by Ellis and Kneser in their study of the absorption of light by liquid oxygen [16]. Details of molecular oxygen spectra were discussed in later reviews [17,18]. More recent studies have reported the same spectral absorption peaks for molecular oxygen dissolved in solvents [19,20,21].

The aim of the present study was to demonstrate that examining the spectral properties of the Raman diffraction pattern enables access to details of the Raman photoexcitation of oxygen molecules. In particular, we observed Stokes components corresponding to the excitation of the two lowest vibrational sublevels of the ¹O₂ state in agreement with previous absorption studies. The novelty lies in the fact that the peaks are Stokes signals in the visible spectrum, generated during the excitation of molecular oxygen via a stimulated Raman process. The presence of both peaks confirms the oxygen origin of the reported Stokes signals, since only the oxygen molecule can exhibit such a spectral response.

As a second aim of the work, we measured the dependence of the Stokes signals’ pump energy. We demonstrated exponential gain in the Stokes signals due to Raman ¹O₂ photoexcitation, confirming basic predictions of the standard stimulated Raman excitation theory [22,23,24,25].

2. Theoretical Considerations

Figure 1 shows the schematic of the first two vibrational sublevels, ¹O₂(0) and ¹O₂(1), of the ¹O₂ electronic state, as established by the studies cited above. ¹O₂(0) lies 0.97 eV from the ground, which corresponds to the energy of a photon of 1270 nm in wavelength. ¹O₂(1) is at 1.16 eV, which corresponds to the energy of a photon of wavelength 1067 nm. Both states have rotational sub-levels that are not indicated in the figure. Raman excitation generates Stokes signals for these two levels. For example, when pumping at 411 nm, Stokes signals at 607 and 668 nm are expected. Anti-Stokes components are also generated in the ultraviolet (UV) region of the spectrum, but this contribution is generally small for the pulse energies used in the experiments.

Stimulated Raman corresponds to the interaction between Stokes and anti-Stokes photons with molecular involvement. Basic theory of the effect shows that there are two kinds of stimulated Raman scattering signals: one set of signals propagates in the direction of the excitation beam, and another set forms an angle with respect to this direction [24,25]. As a result, the stimulated Raman interaction-induced diffraction pattern shows a central spot and concentric rings [14].

As discussed in the Introduction, the Stokes signal experiences exponential gain with increasing propagation distance, while the anti-Stokes beam exhibits losses. In the approximation of a large constant pump field, the gain is [25]

$$g({\lambda }_S)={\displaystyle \frac{12\pi }{{\lambda }_S}}{\displaystyle \frac{{\chi }_R^{(3)}\left({\lambda }_S\right)I_P}{n_s^2({\lambda }_S)c{\epsilon }_o}},$$

(1)

where ${\lambda }_S$ is the Stokes wavelength, ${\chi }_R^{(3)}({\lambda }_S)$ is the third-order Raman susceptibility at ${\lambda }_S$, $n(\lambda {}_S)$ is the refraction index at ${\lambda }_S$, $I_P$ is the pump intensity, $c$ is the speed of light, and $\epsilon {}_o$ is the vacuum permittivity.

The ring Raman diffraction pattern ring’s structure corresponds to the phase-matching condition given by the following equation [13]

$$\Delta \overrightarrow{K}=2{\overrightarrow{K}}_P-({\overrightarrow{K}}_S+{\overrightarrow{K}}_a)={\displaystyle \frac{4\pi n({\lambda }_P)}{{\lambda }_P}}-{\displaystyle \frac{2\pi n({\lambda }_a)}{{\lambda }_a}}-{\displaystyle \frac{2\pi n({\lambda }_S)}{{\lambda }_S}}=0,$$

(2)

where ${\overrightarrow{K}}_P$, ${\overrightarrow{K}}_S$, and ${\overrightarrow{K}}_a$ are the wavevectors of the pump, Stokes, and anti-Stokes fields, respectively, ${\lambda }_P$ and ${\lambda }_a$ are the pump and anti-Stokes wavelengths, respectively, and $n({\lambda }_P)$, $n({\lambda }_a)$, and $n({\lambda }_S)$ are the refraction indexes at ${\lambda }_P$, ${\lambda }_a$, and ${\lambda }_S$, respectively. Equation (2) defines the propagation angle of these Stokes components. Additional deviation can take place due to the nonlinear refraction induced by the same Raman effect [13]. As indicated by equation (2), the radii of the rings depend on the wavelength of each of the fields. Thanks to this, the procedure for separating and identifying different Stokes components becomes less cumbersome. In the Experimental Section, we take advantage of this fact, allowing us to identify the Stokes signals corresponding to the Raman transition to the ¹O₂ electronic state vibrational sublevels (0) and (1).

3. Materials and Methods

Figure 2 shows a schematic of the experimental setup. An optical parametric oscillator (OPO, OPOTEK, Carlsbad, CA, USA) provides the pump beam. The device generates 5-nanosecond pulses with an average energy of 20 mJ at 10 Hz. We used a pump wavelength of 411 nm. A 20-cm-focal-length lens focused the pumping light onto the sample. We estimated a waist beam radius of 5.2 mm and a Rayleigh range of 20 cm. The sample was distilled water in a 20-cm path-length glass cuvette. A long-pass filter (LPF) selects light matching the ¹O₂ Stokes contribution. We used an LPF with a cut-off wavelength of 600 nm. The filter removed all Stokes contributions from the solvent molecules, leaving only those from dissolved oxygen molecules [9,10]. As a result, a red-colored Raman diffraction pattern with a central spot and a ring appeared at the cuvette’s exit. Neutral density filters (NDFs) were used to control the intensity of this light and prevent saturation of the detection system. The pattern was analyzed using a spectrometer (Ocean Optics USB 2000, Orlando, FL, USA) connected to an optical fiber (OF). We placed the optical fiber’s entrance coupler 12 cm from the pump waist position. At that distance, the ring’s radius was about 11 mm and the coupler’s entrance diameter 1 mm.

We collected spectra from different parts of the diffraction pattern by scanning the fiber’s coupler along the center of the pattern in 1-mm steps. We also measured the signals’ pump-energy dependence at the central spot and the ring, using a diode detector (DET100 A2, Thorlabs, Newton, NJ, USA) and varying the pump energy with neutral density filters placed in front of the cuvette. We used a set of filters with optical densities from 0.1 to 1.9. Pump pulse energy was calibrated using a pulse energy detector (Pulsar-1, Newport Corporation, Ophir, North Logan, UT, USA).

4. Results and Discussion

Figure 3a shows a photograph of the diffraction pattern observed when pumping at 411 nm and using a long-pass filter with a cut-off wavelength at 600 nm. The pattern shows the central spot and the ring structure. Previous experiments demonstrated that the observed signal corresponds to the Stokes signal generated by the Raman oxygen excitation toward the ¹O₂ level [9,10,11,12]. Using the spectrometer, we collected spectra along the direction indicated by the white arrow as we scanned the optical fiber coupler with 1-mm steps. For each spectrum, we determined the Stokes signal amplitude as a function of the positions at 607 and 668 nm.

Figure 3b shows this dependence on a semi-logarithmic scale. The blue crossed squares correspond to the signal at 607 nm, while the red crossed circles refer to that at 668 nm. The signal at 668 nm is generally smaller than that at 607 nm, particularly for the central spot (point “A”). In some points closer to the ring - indicated by “B” and “C”- the signals become comparable. Figure 3b shows that the ring radii depend slightly on wavelength, allowing separation of Stokes components with slightly different wavelengths.

Figure 4 shows the spectra measured on all three points “A”, “B”, and “C” (Figure 4a, Figure 4b and Figure 4c, respectively). All signal amplitudes are in the same arbitrary units. The signal at point “A” is more than two orders of magnitude larger than that at “B” and “C”. The most important feature is that the central spot exhibits a single peak centered around 607 nm, while that at “B” and “C” shows a double-peak structure with peaks around 607 and 668 nm. The spectra confirm the generation of Stokes signals corresponding to Raman transitions from the ground state to the ¹O₂(0) and ¹O₂(1) levels. Only the oxygen molecule can exhibit the double-peak response in the Stokes signal spectrum, as shown in Figure 4b and Figure 4c. For the central spot, the contribution at 668 nm is more than one order of magnitude smaller than the Stokes signal at 607 nm. Some spectral asymmetry toward longer red wavelengths is evident in Figure 4a due to the contribution at 668 nm.

We also studied the dependence of the Stokes signal on the pump energy. We used the set of neutral optical filters to change the pump energy. Figure 5 shows this dependence on a semi-logarithmic scale measured for the central spot (5a) and the ring structure (5b). Both dependencies are similar. The pulse energy changes by a factor of 20, while the Stokes signal changes by 5 orders of magnitude, indicating exponential growth.

There is a reduction in exponential growth for energies above 8 mJ. The blue solid line in both plots, Figure 5a and Figure 5b, corresponds to exponential growth with a gain of 85 m^-1, while the green line corresponds to an energy gain of 21 m^-1. At high intensity, the coupling between Stokes and anti-Stokes components increases. Because the anti-Stokes components exhibit absorption, an additional loss channel for the Stokes component limits the gain. For the conditions of our experiments and the maximal pulse energy of (17±2) mJ, we estimate an average pump intensity of (1.5±0.2) 10¹⁶ W/m2. Using equation (1), we determined a ¹O₂ Raman susceptibility of (0.42 ± 0.05) × 10^-24 m² V^-2.