I am just answering because I myself did exactly that.
I did a Master's degree & thesis on theoretical high-energy physics (some B meson decay at CERN...) and then a purely experimental PhD building a lab for a cold atom trapping machine.
So yes. It's possible.
First year PhD students are generally not supposed to have any "work" experience in that specific field. There are some countries, e.g. Germany, where a PhD student has usually already done many internships (some of them along side their studies during term time) within the group that eventually hosts their PhD. Also in Germany, your courses become fully elective and specialised so that you can essentially only study the subject that you'll end up doing your PhD in. In the UK (my case) on the other hand, this is not quite the case. You can do 2 month summer internships and maybe a final year project, but nothing "long term" where you already start working alongside more senior Lab members helping them in the day-to-day operations that may not immediately help your thesis/report.
Just accept that you will have a somewhat steeper learning curve than, say, other PhD candidates that have already undergone coursework, experiences, and trainings in that specific field. As @Elodin said, check with the PI of the Lab if they would even accept someone with your background, because maybe student supply is so high that they would only consider people that can hit the ground running. This will depend on the size of the group and on the amount of immediate work that needs to be done, and potentially on how many PhD/Postdocs there are that are "free" to train you (i.e. not busy writing PhD theses or applying for jobs). Hence why you should ask the PI in question.
Just make it clear that you are motivated and determined to learn. And that your background will, eventually, make you even more valuable than a pure experimentalist (speaking from experience). Back in my day, I went on a two week internship in my PI's old group in Germany to get some work experience in the field, e.g. how to use a laser for the first time. This was not part of my degree, so I didn't have any pressure to "produce" results, and I was a sponge trying to absorb jargon, techniques, principles, that I could go on and read about later on. I had food & accommodation paid for, so I only really "volunteered" my time. If you wish, you could try and volunteer for a similar opportunity to see if that would bolster your chances.
