# What is the temperature of an ultra-cold Rydberg plasma?

## TL;DR

This study measures the initial electron temperature of ultra-cold Rydberg plasmas by analyzing their expansion velocities, revealing how it depends on Rydberg density and supports the existence of a spectral bottleneck.

## Contribution

It provides the first quantitative measurements of electron temperature in ultra-cold Rydberg plasmas and links it to plasma environment and Rydberg state spectrum.

## Key findings

- Electron temperature depends mainly on plasma environment at decoupling.
- The results support the existence of a Rydberg state spectral bottleneck.
- Electron temperature varies with Rydberg atom density and binding energy.

## Abstract

We have measured the asymptotic expansion velocities of ultra-cold plasmas (UNPs) which evolve from cold, dense, samples of Rydberg rubidium atoms using ion time-of-flight spectroscopy. From this, we have obtained values for the initial plasma electron temperature as functions of the original Rydberg atom density and binding energy. Our results show that the electron temperature is determined principally by the plasma environment when the UNP decouples from the Rydberg atoms, which occurs when the plasma electrons become too cold to ionize the remaining Rydberg population. Furthermore, the dependence of the electron temperature on Rydberg atom density gives strong indirect evidence for the existence of a bottleneck in the spectrum of Rydberg states that coexist with a cold plasma.