# Two-Color X-Ray Coherent Diffraction Imaging of Helium Nanodroplets

## TL;DR

This paper demonstrates a novel two-color X-ray diffraction technique to image helium nanodroplets with femtosecond temporal resolution, effectively separating overlapping images using energy sensitivity and pattern recognition.

## Contribution

It introduces a method to disentangle simultaneous two-color X-ray diffraction images of nanoscale particles, validated on helium nanodroplets, enabling ultrafast structural imaging.

## Key findings

- Successful separation of two-color diffraction images using energy-sensitive detection.
- Validation of the method through Mie theory fitting of spherical helium nanodroplets.
- High agreement between experimental data and theoretical models, especially in outer regions.

## Abstract

Two-color X-ray imaging with Free Electron Laser pulses offers a powerful approach for probing ultrafast structural dynamics in nanoscale systems, combining (near-)atomic spatial resolution with femtosecond temporal precision. The first X-ray pulse captures the object's initial state, while the time-delayed second pulse records its subsequent evolution. A key challenge lies in disentangling the two views simultaneously recorded by the same detector. We demonstrate the realization of this approach on structurally varying nanoscale particles using two X-ray pulses of different photon energies, 1 and 1.2~keV. Sub-micrometer helium nanodroplets generated in vacuum are irradiated by the two X-ray pulses separated in time by up to 750 femtoseconds. Taking advantage of the high energy sensitivity of the imaging detector, we separate the overlapping images by analyzing individual pixel counts and applying pattern recognition. The helium nanodroplets' spherical shape allows us to cross-validate this approach by fitting the radial scattering profiles with Mie solutions for a bichromatic field. The excellent agreement between the two methods, particularly in the sparsely illuminated outer regions of the diffraction images where fine structural information is encoded, highlights the quality of this approach and its potential for future advanced X-ray movie techniques.