# Conformer-selective Photoelectron Circular Dichroism: Experimental Development and Application to Nitrogen Chirality

Etienne Rouquet, Julien Vincent, Valéria Lepère, Lionel Poisson, Anne Zehnacker

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Etienne Rouquet, Julien Vincent, Valéria Lepère, Lionel Poisson, Anne Zehnacker. Conformer-selective Photoelectron Circular Dichroism: Experimental Development and Application to Nitrogen Chirality. Structural Dynamics, In press. hal-05340266

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Distributed under a Creative Commons Attribution 4.0 International License
Conformer-selective Photoelectron Circular Dichroism: Experimental
Development and Application to Nitrogen Chirality

Etienne Rouquet, a,b) Julien Vincent, a) Valéria Lepère, a) Lionel Poisson, a) Anne Zehnacker a)*

a) Institut des Sciences Moléculaires d'Orsay (ISMO), CNRS, Université Paris-Saclay,
F-91405 Orsay, France

b) Synchrotron Soleil, L'Orme des Merisiers, St. Aubin BP48, F-91192 Gif sur Yvette, France

*Corresponding author: anne.zehnacker-rentien@universite-paris-saclay.fr

ABSTRACT

Here we describe recently developed upgrades to our experimental scheme for obtaining the photoelectron spectrum (PES), the anisotropy parameter, and the PhotoElectron Circular Dichroism (PECD) of jet-cooled flexible molecules with conformer selectivity. The one-color Resonance-Enhanced Multi-Photon Ionization (REMPI) process used allows ionizing selectively the different conformers present in the supersonic expansion by selecting their S₀-S₁ transition. We first describe the experimental set-up with emphasis on the data acquisition and processing. Then we apply this ionization scheme to a flexible molecule, 1,2,3,4-tetrahydro-3-isoquinoline methanol (THIQM). This molecule shows two stereogenic centers, namely, an asymmetric carbon and a nitrogen atom. It exists in two conformers, THIQMᵢ and THIQMᵢᵢ, which differ by the direction of the intramolecular hydrogen bond and the absolute configuration of the nitrogen atom. Therefore, these two conformers are also diastereomers, endowed with slightly different ionization energies. The ionization energy of THIQMᵢ, which shows an OH...N hydrogen bond, is slightly higher than that of THIQMᵢᵢ. Their PECD spectra, although of identical signs, differ in shape and magnitude. Surprisingly, the anisotropy parameter is more sensitive than the PECD to the conformational isomerism at play in this system.
Introduction

Chirality is often encountered in organic molecules, in particular those related to the biosphere. It
can originate from the presence of one or several stereogenic centers, as observed in amino acids or
sugars, from hindered rotation around a single bond, or from a helical arrangement of the atoms.
Several stereogenic centers exist besides the asymmetric carbon atom, i.e. a carbon atom
substituted by four different substituents. Examples encompass phosphorus¹ or nitrogen atoms,
which are also chiral due to their tetrahedral structure. Quaternary amines with four different
substituents are permanently chiral ions. Similarly, neutral amines in which the umbrella motion,
which would result in racemization, is locked, are permanent chiral centers. Locking the nitrogen
inversion can be obtained by including the nitrogen atom into a ring, as historically observed in
Tröger's bases. Nitrogen-based stereogenic centers are also encountered in cyclic amino acids² or in
cinchona alkaloids.³

In addition to the umbrella motion related to chirality, nitrogen-containing cyclic systems like those
based on piperidine, morpholine, or tetraisoquinoline, show ring inversion that adds to the
complexity of the conformational landscape. Molecules with such a complex potential-energy surface
provide a strigent case for chiroptical spectroscopy due to the sensitivity of these methods to
conformation.⁴ ⁵⁻⁷ Among them, valence-shell PhotoElectron Circular Dichroism (PECD) is very well
suited for gas-phase studies due to the intensity of the measured asymmetry.⁸⁻¹⁸ PECD is defined as a
backward/forward asymmetry in the angular distribution of the electrons resulting from the
ionization of a randomly oriented chiral molecule by Circularly Polarized Light (CPL), along the light
propagation axis.¹⁹⁻²¹ It is an intense effect because it is allowed in the electric dipole approximation.
The high sensitivity of PECD to conformational isomerism¹⁴,²² has been observed in one-VUV-photon
experiments conducted on the DESIRS beam line of the French Synchrotron SOLEIL.¹⁶,²³ An alternative
scheme is non-resonant multiphoton ionization resting on a femtosecond laser²⁴⁻²⁸ but their large
spectral bandwidth precludes conformation selection. Schemes based upon nanosecond Resonance-
Enhanced MultiPhoton Ionization (REMPI), with intrinsic high resolution capabilities, have also been
reported in the past decade.²⁹,³⁰ Recently, ionizing selectively each conformer was made possible by
using a REMPI scheme based on nanosecond (ns) lasers.³¹

PECD measurements often rest on a Velocity Map Imaging (VMI) spectrometer based on electrostatic
focusing of the electrons and a position sensitive detector combined with either delay-line anode or
a phosphor screen and camera set-up.³²⁻³⁵ REMPI-based PECD measurements are experimentally very
different from those obtained with synchrotron radiation. In the latter, the high repetition rate
allows coincidence measurements and less than one event per pulse is desirable, making the use of a
delay-line anode recommended. The consequence is that the acquisition proceeds in a "single event"
mode, making error evaluation by a Poisson distribution possible. In the REMPI scheme presented
here, the low repetition rate of the ns lasers (10 Hz) prevents this approach. The larger intensity per
light pulse results in the simultaneous detection of several electrons, each of them leading to a light
flash on the camera. The challenge is thus to acquire the data such as every electron is counted once,
and only once, so that error evaluation using a Poisson distribution is possible, preferentially
reaching "real time" acquisition at a frequency of 10 Hz allowing for an acquisition time of a few
hours for statistically significant results.

Here, we describe the experimental set-up used for measuring laser-based one-color REMPI PECD,
with special emphasis on the data acquisition process. We then apply this scheme to a test molecule,
namely, 1,2,3,4-tetrahydro-3-isoquinoline methanol (THIQM, Figure 1). THIQM was chosen as a test
case for this study for several reasons. First, it is a benzene derivative, endowed with a S<sub>0</sub>-S<sub>1</sub>
transition in the near UV (~ 4.5 eV) and an ionization energy below 9 eV, which makes 1-color 2-
photon ionization possible. Second, the nitrogen atom is a stereogenic center leading to nitrogen-
centered chirality.

In substituted cyclic systems like THIQM, the substituent may adopt axial or equatorial positions,
resulting in different conformers. The equatorial position is energetically favored, especially for bulky
substituents. For small substituents, the axial and equatorial conformers coexist, as observed in jet-
cooled indane derivatives.<sup>36-40</sup> THIQM follows this trend: only the conformer with equatorial CH<sub>2</sub>OH
is observed but the axial or equatorial positions of the NH hydrogen coexist under supersonic
expansion conditions, like in the parent molecule tetrahydroisoquinoline.<sup>41,42</sup> This results in the
coexistence of two stable conformers, namely, THIQM<sub>I</sub> and THIQM<sub>II</sub>. They differ by the nature of
their intramolecular hydrogen bond, OH...N for THIQM<sub>I</sub> and NH...O for THIQM<sub>II</sub>. Different coupling
between the nitrogen lone pair (LP) and the benzene π orbital are thus expected for the two
conformers, as described for the proline amino acid, for which different photoelectron (PES) and
PECD spectra were observed.<sup>16</sup> The axial or equatorial positions of the NH hydrogen correspond to
opposite absolute configurations of the nitrogen stereogenic center. THIQM<sub>I</sub> and THIQM<sub>II</sub> are
therefore diastereomers.

**Experimental methods**

The experimental scheme described here rests on a one-color two-photon ionization process *via* a
REMPI scheme already described.<sup>38,43</sup> The two photons arise from the same laser pulse; they have
therefore identical energy and polarization. Briefly, the energy of the photon is scanned in the range
of the S<sub>0</sub>-S<sub>1</sub> transition. Ionization only happens for resonant excitation of a S<sub>1</sub> vibronic state, which
allows recording the spectrum of the S₀-S₁ transition by monitoring the ion or electron signal as a function of the photon energy. Due to the exquisite sensitivity of the S₀-S₁ transition to the conformation, different conformers of a given molecule are excited hence ionized at different photon energies. The combination of the low temperature achieved in the jet and the resolution (0.02 cm⁻¹) of the ns laser used allows resolving transitions separated by a few cm⁻¹. Setting the laser wavelength on the different bands observed in the S₀-S₁ spectrum thus allows recording conformer-specific PECD.

**Experimental set-up:** The experiment was based on a pulsed supersonic expansion of helium at a pressure of 2 bar, seeded with the molecule of interest and expanded through a 100 µm nozzle (General Valve). (R) THIQM from Santa Cruz Biotechnology, purity>98%, or (S) THIQM from Sigma-Aldrich, purity>97% (enantiomeric purity not mentioned), was contained in an oven prior to the expansion and heated at 60°C. The jet was then skimmed (skimmer of 1490 µm) and interacted with the CPL in the interaction zone of a VMI spectrometer (Photek ©). The emitted electrons were detected by MicroChannel Plates (MCP) combined with a phosphor screen and a camera.

**Lasers and polarization control:** The CPL was provided by a frequency-doubled dye laser (Sirah equipped with Coumarin 540A, barium borate (BBO) doubling crystal) pumped by the second harmonic of a Nd-YAG laser (Surelite III – Continuum). Most of the visible light was removed by a low-pass UG5 filter. The UV laser was then further cleaned from the remaining visible light and UV super radiance by a MgF₂ Rochon prism R<sub>exc</sub>, resulting in a linearly polarized UV laser (horizontal polarization). Circular polarization of the UV laser was then obtained using an achromatic quarter-wave plate (λ/4)<sub>exc</sub> (Thorlabs, AQWP05M-340) mounted on a computer-controlled motorized rotation stage (Thorlabs, PRM1Z8 rotation stage with KDC101 motor controller), set at ± 45° of the fast axis for right and left CPL, respectively, using the convention that the observer is facing the light propagation. The rate of circular polarization was measured following the published procedure that uses an analyzer composed of a second rotating quarter-wave plate (λ/4)<sub>ana</sub> followed by a Rochon prism R<sub>ana</sub>, and a photodiode.<sup>44</sup> Briefly, the procedure consists in first defining the neutral angles of (λ/4)<sub>exc</sub> using the two Rochon prisms aligned such as the transmission of the horizontal polarization, that of the excitation light, is optimized. Then the angle φ defined as the angle between the vertical axis (laboratory frame) and the fast axis of (λ/4)<sub>ana</sub> is varied by steps of 22.5°, i.e. 8 steps for a full revolution of the plate. This measurement was repeated twice, averaging the signal on 50 laser shots. The transmitted intensity, defined as a function of the four Stokes parameters S<sub>n=0-3</sub>, was then plotted as a function of φ (Equation 1).

$$I(\varphi) = \frac{1}{2}(S_0 + S_1 \cos^2 2\varphi + S_2 \cos 2\varphi \sin 2\varphi + S_3 \sin 2\varphi) \quad (1)$$
The Stokes parameters describe the polarization state of the light and are defined as the total intensity $S_0$, the difference between the vertical and horizontal polarization intensities $S_1$, the difference between the linear polarization at +45° and -45° intensities $S_2$, and the difference between the right CPL (RCPL) and left CPL (LCPL) intensities $S_3$. The dependency of the transmitted intensity as a function of $\phi$ is shown in Figure 2. Analysis of the obtained curve allowed defining a CPL ratio $S_3/S_0$ better than 99.5 % $\pm$ 0.7%.

**Detection of the electrons:** The produced electrons were extracted and accelerated by grid-less electrodes with the repeller and extractor voltages set at $V_{rep}$ = -500 V and $V_{ext}$ = -323 V, respectively. The $V_{ext}/V_{rep}$ ratio was chosen to optimize the resolution as described by Eppink and Parker in their seminal work.⁴⁵ The optimal ratio depends on the set-up geometry and the shape of the electrodes used. Here, this ratio was 0.646. The accelerated electrons were then collected by a pair of position-sensitive MCP (diameter 40 mm) set at a voltage of 1900 V (gain of ~ $10^6$) and a phosphor screen (P43) with rise and decay times of 0.5 and 1.3 ms, respectively. The luminescence emitted was focused on and observed by a computer-controlled Complementary Metal Oxide Semiconductor (CMOS) camera (IDS Imaging, UI-3060CP-M-GL Rev.2). The energy calibration was obtained by measuring the radius of the photoelectrons image produced by 1,4-diazabicyclo[2.2.2]octane (DABCO) excited at an energy of 266.69 nm as a function of $V_{rep}$, keeping the $V_{ext}/V_{rep}$ constant and using the energy conservation law $E_{cin} = \frac{|q| \cdot V_{rep} \cdot r^2}{C^2}$. The ultimate resolution was not measured but, at the used voltages, the typical width of the HOMO band in the PES spectrum of DABCO is 0.2 eV FWHM.⁴⁶ ³²

**Acquisition process and software:** PECD measurements rest on the subtraction between successive images obtained using LCPL and RCPL, respectively (vide infra). To get rid of the long-term signal variations due to drift of the sample temperature, hence concentration, or fluctuation in the laser power, the position of the computer-controlled motor defining the angle of $(\lambda/4)_{exc}$ is periodically changed after the acquisition of a pre-defined number of electron impacts corresponding to an accumulation time of ~ 2 to 5 min for each polarization. A statistically-significant PECD measurement requires 15 to 40 pairs (LCPL and RCPL images), resulting in a total acquisition time of ~ 1 to 6 hours. The experiment generates images at the repetition rate of the laser (10 Hz) and each image contains numerous spots due to electron impacts (typically a few tens). A single electron impact on the MCP gives rise to a spot of about 10 to 20 pixels on the camera. The challenge here is to count each electron impact as a single event, avoiding the errors due to the superposition of events in a limited zone, while keeping a total accumulation time below a few hours for each probed transition. This is
usually done by centroid analysis. Summing the images before processing results in errors because of
the accumulation of electron impacts in the same zone, each resulting spot being counted once even
if it corresponds to several electron impacts. This error results in the underestimation of the PES
intensity in the region of an intense signal, as shown in Figure 3 when we use a program "oldPCO"
that does not consider this problem. This can be partially solved by decreasing the laser power, but it
results in too long an accumulation time (~18h). We must therefore record, process and sum all the
images in real time during the measurements. To do so, we have developed a new software (called
newPCO) based on a previously developed version,⁴⁷ which we have modified and adapted to parallel
calculations on GPU using OpenCL. This software allows acquiring images at a repetition rate of 10 Hz
and processing them in real time, without losing information and without decreasing the signal
intensity. The process rests on the complementary use of CPU and GPU and is shown in Figure 4,
together with the typical images obtained at each step of the acquisition process.

The whole acquisition consists in averaging an identical number N/2 of accumulated images alternatively recorded with either RCPL or LCPL, with N being an even number, to get the total PES and PECD data. Each of the accumulated images, denoted as Pᵢ, with i=1 to N, is the sum of Qᵢ processed images, Qᵢ being the number of laser shots necessary to reach a count rate initially defined.

To do so, the first step is an initialization stage. The scripts are transferred to the GPU, on which
memory is allocated for later use in the second loop (Loop₂). The four angles of (λ/4)exc
corresponding to either RCPL or LCPL, defined in the polarization control step, are introduced as
input parameters in the software. The region of interest (ROI) on the detector is selected as a circle
slightly larger than the zone containing the emitted electrons with the expected kinetic energy, i.e.
the actual impacts that must be processed. The number of electron counts that must be accumulated
within the ROI for each of the N images is also defined during the initialization stage. The percentage
P of the mean value of the signal used for defining the threshold, typically 99.7 %, is set as a
parameter

Then two intertwined loops are run. The first one, Loop₁, runs from i=1 to N. It consists in changing
the angle of (λ/4)exc for defining the direction, R or L of the CPL, and storing the resulting
accumulated N images on the CPU. Loop₂ runs from j=1 to Qᵢ at a frequency of 10 Hz, i.e. for each
laser shot. It combines both CPU and GPU communicating with OpenCL and consists of two scripts,
Script₁ and Script₂, with respective allocated memories M₁ and M₂ on the GPU. A third script, Script₃,
runs in parallel with Script₁ on the CPU. At each laser shot, the raw image (1936 x1216 pixels) is
transferred from the camera to the CPU and is processed by Script₃, which plots a histogram of the
values of all the pixels. The parameters of the resulting Gaussian curve (mean value and standard
deviation) are stored and used to define the threshold Tj. Determination of Tj is done dynamically by
multiplying the mean value of the Gaussian distribution of the pixels defined by Script₃ by the value P
defined during the initialization step. Simultaneously, the raw image is transferred to M₁ and is
processed by Script₁ that cleans off the hot and dead pixels by inspecting a 5x5 matrix around each
pixel, all the pixels being inspected in parallel. If the selected pixel is 10 times more intense than the
average value within the 5x5 matrix, it is considered as a hot pixel and is replaced by the average
value. The cleaned image is then transferred to M₂ and is processed by Script₂, which is a centroid
analysis. The value of each pixel is compared to the values of the adjacent pixels within a 9x9 matrix.
The pixel is taken into consideration and is set to 1 if and only if it fulfills the two conditions, namely,
to be the maximum in the 9x9 matrix and 1/3 of the pixels of the matrix have a value larger than the
threshold Tj. If not, it is set to zero. This results in the creation of a processed image. This centroid
analysis yields a resolution of 4 pixels and avoids being limited in electron flux by saturation effects: if
two impacts are separated by 4 pixels, they can be distinguished. The spots separated by less than 4
pixels from the edge of the detector are not considered. The processed Ij image generated by Script₂
is then added to the summed I₁ to Ij-1 images and Loop₂ is repeated iteratively till the number of
electron counts that must be accumulated is reached.

The use of GPU for all the calculations necessary for the image analysis considerably reduces the
signal acquisition time. While only 12 threads could be executed simultaneously on the CPU when
using the previous software, almost 50000 threads could be run in parallel on the 24 Streaming
Multiprocessors (SM) of the GPU (RTX 4060 NVIDIA). Allocation of M₁ and M₂ is done only once
before Loop₁ starts. In terms of Loop₂, the limiting step is the transfer of data between the CPU and
the GPU (transfer of the raw image and reception of the processed image). Still, the acquisition and
processing time for one image is of the order of 80 ms, i.e. less than the 100 ms separating two laser
pulses. This new acquisition software makes it possible to reduce the acquisition time by
approximately a factor of 10 because of the faster acquisition process due to the use of GPUs and the
centroid procedure that makes it possible to work at larger count rate (see Figure 2). The last step is
the analysis of the N accumulated images to retrieve the PES and PECD data using the inverse Abel
transform as implemented in the PBasex software.⁴⁸ The analysis includes defining error bars based
on Poissonian noise analysis, which is made possible by this acquisition procedure.

Results

1) Structure of THIQM

We shall first briefly recall the structural parameters of THIQM.⁴¹ THIQM has two chiral centers. The first one, which makes the molecule permanently chiral, is the substituted carbon atom in the alicyclic ring. This results in the two enantiomers (R) and (S) THIQM. Moreover, the nitrogen atom is a
transient chiral center, depending on the equatorial or axial position of its lone pair. For each (R) and (S) enantiomer of THIQM, two diastereomers are thus expected. Two intense bands appear indeed in the S₀-S₁ spectrum. They have been assigned to two conformers, namely THIQMᵢ and THIQMᵢᵢ, on the basis of IR-UV double resonance experiments combined with quantum chemistry calculations.⁴¹ Both conformers have their CH₂OH substituent in equatorial position. In the most stable of them, namely THIQMᵢ, the nitrogen lone pair is in equatorial position and acts as a hydrogen bond acceptor from the CH₂OH hydroxyl ((S) configuration of N for (S) THIQM). In THIQMᵢᵢ, the nitrogen lone pair is in axial position and N-H acts as a hydrogen bond donor to the CH₂OH oxygen atom ((R) configuration of N for (S) THIQM). The ion corresponds to the removal of an electron from an orbital mainly localized on the aromatic ring (π orbital). IR spectroscopy of the ionic state shows that its most stable calculated structure is similar to THIQMᵢᵢ and that the THIQMᵢ ion isomerizes to the THIQMᵢᵢ ion.

## 2) REMPI spectrum

The REMPI spectrum of the S₀-S₁ transition, obtained by monitoring the total electron yield as a function of the wavelength, is presented in Figure 5. The two main transitions located at 36896 cm⁻¹ and 36930 cm⁻¹ are assigned to the transition origins of the two most stable conformers, namely, THIQMᵢ and THIQMᵢᵢ, respectively.⁴¹ For the sake of clarity, these two transitions will be called 0₀ᵢ and 0₀ᵢᵢ in what follows. The spectrum shown in Figure 5 is identical to that obtained by monitoring the ion current, despite a slightly lower signal to noise ratio due to the lack of mass selection.⁴¹ However, adding a skimmer in the experiment described here results in colder molecules than obtained in the previous work.⁴¹ In particular, the high-energy shoulder of the 0₀ᵢ transition that was previously observed now appears as a distinct band.

## 3) PES and PECD spectra

The distribution I⁽ᵏ⁾(θ) of the emitted electrons is written as a function of the radius r and the angle θ between emission direction and the light propagation axis, as the sum of Legendre polynomials Pₖ given in Equation (2), with k=1 to n, where n is the number of photons involved in the ionization process.

$$I^{\{p\}}(r, \theta) = I_{0}^{\{p\}}(r) \left(1 + \sum_{k=1}^{2n} b_{k}^{\{p\}}(r) P_{k}(\cos\theta)\right) \quad (2)$$

p defines the light polarization, with p= 0, +1, and -1 for linear, left- and right-handed circular polarization, respectively. The I₀ coefficient is isotropic and is the photoelectron spectrum (PES). Because the intensity of the S₀-S₁ transition varies within a few percent between RCPL and LCPL, due to electronic circular dichroism, the I₀ coefficient is slightly dependent on the helicity of light.⁴⁹,⁵⁰ However, our PECD definition (see below) involves normalization by I₀, which allows overcoming
laser intensity fluctuations or slight variations in sample concentration, as well as the dependency of ionization efficiency upon polarization. The $b_2^{\pm 1}$ coefficient is related to the (achiral) anisotropy parameter in photoionization usually labelled $\beta$, with $b_2^{\pm 1} = -\frac{1}{2} \beta$. The odd terms of this equation, $b_1, b_3$, etc., vanish for linear polarization or non-chiral molecules.

In the case of two-photon ionization, the normalized distribution becomes:

$$I^{[p]}(r, \theta) = I_0^{[p]}(r)(1 + b_1^{[p]}(r)P_1(\cos \theta) + b_2^{[p]}(r)P_2(\cos \theta) + b_3^{[p]}(r)P_3(\cos \theta) + b_4^{[p]}(r)P_4(\cos \theta)) \quad (3)$$

$b_1^{[1]} = -b_1^{[-1]}$ and $b_3^{[1]} = -b_3^{[-1]}$ so that opposite effects are observed for right and left CPL.²³,⁵¹ Thus, by subtracting the electron angular distribution obtained with left and right CPL, the even terms cancel out, and by integrating over only one hemisphere of the image, comes the following definition of PECD.²⁹,³⁰

$$\text{PECD}(r) = 2b_1^{[1]}(r) - \frac{1}{2}b_3^{[1]}(r) \quad (4)$$

$b_1^{[1]}$ and $b_3^{[1]}$ have opposite signs for opposite enantiomer, and so also does PECD, such that a mirroring relation is expected for the PECD of the two enantiomers of a given molecule.

Figure 6 shows the PES and the $b_2^{\pm 1}$ parameter of the two conformers recorded by setting the laser at their respective transition origin, i.e. $0_0^0_I$ (total ionization energy of 9.150 eV) and $0_0^0_{II}$ (total ionization energy of 9.159 eV) for THIQM$_I$ and THIQM$_{II}$, respectively. The two PES differ in shape, in particular at the ionization onset, defined as the intercept between the rising edge and the energy axis. The ionization energy of THIQM$_I$ is 8.42 ±0.1 eV, while that of THIQM$_{II}$ is much lower, at 8.16 ±0.1 eV. This is in good agreement with the vertical ionization energies calculated at the DFT level at 8.44 and 8.08 eV for THIQM$_I$ and THIQM$_{II}$, respectively. This difference reflects the structural difference between the two conformers and the shape of their HOMO (see Figure 6). In THIQM$_I$, which shows an OH...N hydrogen bond, the contribution of the nitrogen lone pair to the HOMO is larger than in THIQM$_{II}$. The resulting delocalization contributes to the stabilization of the HOMO of THIQM$_I$, which explains its higher ionization energy.

The PES and PECD spectrum of the two conformers recorded by setting the probe on their respective transition origins are shown in Figure 7. For each conformer, an excellent mirroring is observed between the PECD of the two enantiomers. The PECD of THIQM$_I$ and THIQM$_{II}$ are of identical sign (positive for the (R) enantiomer and negative for (S)) but differ in shape. This difference cannot be explained in terms of different total ionization energies provided by the lasers, which is smaller than $10^{-2}$ eV. For THIQM$_I$, the PECD is strongest at the ionization onset (~8%) then decreases to a plateau
at ~3%. THIQM<sub>II</sub> shows a slightly weaker intensity at the onset (~6%) but more marked oscillations. In both cases, the PECD vanishes when the electron kinetic energy is zero and is the strongest at the ionization onset. While the difference in PECD between THIQM<sub>I</sub> and THIQM<sub>II</sub> are limited, the two conformers show very different $b_2^{\{\pm1\}}$ parameters (see Figure 6). That of THIQM<sub>II</sub> is constant over the whole PES spectrum, with a value of 0.1, while that of THIQM<sub>I</sub> is close to zero at the ionization onset, then slightly increases. Different $b_2^{\{\pm1\}}$ parameters were not observed for the two conformers of flexible molecules like 1-indanol or cis-1-amino2-indanol.<sup>23,31,38</sup> In addition, the diastereomer molecules cis-1-amino-2-indanol and trans-1-amino-2-indanol show identical $b_2^{\{\pm1\}}$ parameters.<sup>52</sup> This is because the ionized orbitals were of similar nature for the two conformers or the two diastereomers. In the case of THIQM, the HOMO is slightly different for THIQM<sub>I</sub> and THIQM<sub>II</sub> in particular around the lone pair of nitrogen as mentioned before. This may explain the difference in their $b_2^{\{\pm1\}}$ parameters. In contrast to $b_2^{\{\pm1\}}$, $b_4^{\{\pm1\}}$ is very similar for the two conformers and is very weak, as shown in Figure 6.

### 4) Alignment effects

Rotational molecular constants of THIQM are of the order of 1 to 4 GHz, which involves that rotational dephasing mostly destroys alignment in the $S_1$ state prior to the second photon absorption and ionization when using pulses of the order of 10 ns as done in this work. However, alignment resulting from quantum effects have been observed in REMPI experiments based on linearly polarized ns lasers.<sup>53,54</sup> Although these effects are supposed to vanish with CPL, anisotropy in Electronic Circular Dichroism (ECD) has been suggested from the variation of ECD along the rotational contour.<sup>55</sup> In PECD, alignment effects would appear as a non-zero $b_3^{(1)}$ term.<sup>56</sup> Figure 7 compares the PECD defined as $2b_1^{(1)} - 0.5b_3^{(1)}$, which is the rigorous definition for a two-photon process, and the PECD defined as $2b_1^{(1)}$, i.e. neglecting the influence of the second photon, for the two enantiomers and the two conformers of THIQM. The two plots are superimposed within the error, which shows that no alignment effects intervene in this system. Such a behavior was already observed in REMPI PECD of 1-indanol or fenchone.<sup>29,31</sup>

## Conclusion

The electron angular distribution resulting from the ionization of the title molecule, THIQM, has been studied using a one-color two-photon REMPI scheme that allows for conformer selection. From an experimental point of view, simultaneous acquisition and real-time processing of the data is made possible, based on a software resting upon GPU and OpenCL. This set-up therefore allows fast (of the order of the hour) acquisition of the images and makes compatible high signal intensity and photon
counting mode that allows errors to be reliably assessed. The two conformers of the THIQM molecule studied here differ by the stereochemistry of the nitrogen atom that results in an OH...N interaction for THIQM<sub>I</sub>, and NH...O for THIQM<sub>II</sub>. As a result, they are diastereomers due to the conformational chirality of the nitrogen atom. As such, they show different ionization energies, resulting from a different coupling between the nitrogen lone pair and the benzene π orbital. Therefore, the PES of THIQM<sub>I</sub> is shifted up in energy relative to that of THIQM<sub>II</sub>. They also show different $b_2^{\{\pm1\}}$ parameters, that of THIQM<sub>II</sub> being constant at 0.1 over the whole electron kinetic energy range. In contrast, that of THIQM<sub>I</sub> is close to zero. The two conformers have similar however distinct PECD spectra. Although of identical sign, they differ in their shape and intensity. In contrast to what was observed in 1-indanol,<sup>23,31</sup> the PES and the $b_2^{\{\pm1\}}$ parameter seem to be more sensitive here to conformation than PECD. PES and PECD spectra provide therefore complementary information about the THIQM molecule, which is the first molecule showing diasteromerism due to conformational chirality on the nitrogen center studied by these techniques.

## Acknowledgments

We thank Dr. Laurent Nahon and Dr. Gustavo Pino for providing the pBasex software and for helpful scientific discussions. We acknowledge the financial support of the projects CONF-PECD and FLEX-PECD by the program "Investissements d'Avenir LabEx PALM" (ANR-10-LABX-0039-PALM). We acknowledge the computing center MésoLUM managed by ISMO (UMR8214) and LPGP (UMR8578), Université Paris-Saclay (France). This work was supported by state funding from the ANR under the France 2030 program, with reference ANR-23-EXLU-0004, PEPR LUMA TORNADO.

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<figure><img src="image_2.png" /><figcaption>**Figure 1:** a) Molecular structure of (S)-(-)-1,2,3,4-Tetrahydro-3-isoquinolinemethanol (S) (THIQM). The chiral center is indicated with a star. Most stable calculated structure of b) Conformer (S) THIQM<sub>I</sub> c) Conformer (S) THIQM<sub>II</sub>. The dotted lines indicate the hydrogen bonds.</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>**Figure 2:** Normalized transmitted intensity ($S_0$ set to 1) of the circularly polarized UV laser as a function of the angular position $\varphi$ of the quarter-wave plate analyzer ($\lambda/4$)$_{ana}$. The dashed lines are the best fit by $I(\varphi) = \frac{1}{2}(1 + S_1\cos^22\varphi + S_2\cos2\varphi\sin2\varphi + S_3\sin2\varphi)$ obtained with $S_1=-0.089$, $S_2=-0.009$, and $S_3=-0.989$ for the LCP (red curve) and $S_1=-0.035$, $S_2=-0.028$, and $S_3=0.995$ for the RCP (blue curve).</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**Figure 3:** Comparison of PES recorded with the first version of the acquisition program, namely OldPCO, at low (purple line) and high (cyan line) laser intensity and with the second version of the acquisition program, namely NewPCO, at high laser intensity (blue line).</figcaption></figure>
<figure><img src="image_5.png" /></figure>
**Figure 4:** Logics of the software used for recording and processing the electron distribution images.
The inserts at the right describe, from top to bottom: Accumulation of an image as it would be
without processing with the NewPCO software. Raw image corresponding to a single laser shot.
Zoom on the centroid procedure in which each large spot is replaced by a single value (orange dot in
the figure). Processed summed image. Difference LCP-RCP raw (left) and Abel inverted (right)
electron images.
<figure><img src="image_6.png" /><figcaption>**Figure 5:** REMPI spectrum recorded in the region of the S₀-S₁ transition origin. The transition origins of the two conformers are denoted by THIQMᵢ at 36896 cm⁻¹ and THIQMII at 36930 cm⁻¹, respectively.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Figure 6:** a) Photoelectron spectrum (full line) and parameter $b_2^{\{\pm 1\}}$ (full triangles - top spectrum) and $b_4^{\{\pm 1\}}$ (empty triangles - bottom spectrum) obtained by exciting the origin transition of THIQM$_1$ (green) and THIQM$_{II}$ (black) at a total ionization energy of 9,150 eV and 9,159 eV, respectively. The tangents (dashed lines) show the ionization energy thresholds. The arrows indicate the energy of the HOMO. The vertical bars represent the vertical ionization energies calculated at the DFT level. b) HOMO orbitals calculated at the ovgf level on the MP2 geometries for Conformer (S) THIQM$_1$ (top) and Conformer (S) THIQM$_{II}$ (bottom) drawn with an isodensity of 0.02. c) Raw images obtained by summing the images recorded with RCPL and LCPL.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Figure 7:** PES (full line) and PECD (symbols with error bars) obtained by exciting the origin transitions of a) THIQM<sub>I</sub> (9,150 eV total ionization energy) and b) THIQM<sub>II</sub> (9.159 eV total ionization energy). The blue and yellow symbols ((R) enantiomer) correspond to PECD defined as 2b<sub>1</sub><sup>{+1}</sup> - 0.5 b<sub>3</sub><sup>{+1}</sup> and PECD defined as 2b<sub>1</sub><sup>{+1}</sup>, respectively. The superposition between the two plots show that the b<sub>3</sub><sup>{+1}</sup> coefficient is negligible. The red and green symbols ((S) enantiomer) correspond to PECD defined as 2b<sub>1</sub><sup>{+1}</sup> - 0.5 b<sub>3</sub><sup>{+1}</sup> and PECD defined as 2b<sub>1</sub><sup>{+1}</sup>, respectively. Please note larger error bars for the blue curves due to shorter measurement times. The inserts show the LCP-RCP raw (right) and Abel inverted (left) electron difference images.</figcaption></figure>